Reinstate + fix "Revert "regcomp.c: Add a constant 0 element before inversion lists" "

This reverts commit de353015643cf10b437d714d3483c1209e079916 which
reverted 533c4e2f08b42d977e5004e823d4849f7473d2d0, thus reinstating it,
plus this commit adds a fix to get it to pass under Address Sanitizer.

The root cause of the problem is that there are two measures of the
length of an inversion list.  One is SvCUR(), and the other is
invlist_len().  The original commit caused these to get off-by-one in
some cases.  The ultimate solution is to only store one value, and
return the other one based off that.  Rather than redo the whole branch,
I've taken an easier way out, which is to add a dummy element at the end
of some inversion lists, so that they aren't off-by-one.  Then the other
patches from the original branch will be applied.  Each will be
tested with Address Sanitizer.  Then the work to fix the underlying
problem will be done.

The original commit's message was:

This commit is the first step to separating the header from the body of
inversion lists.  Doing so will allow the compiled-in inversion lists to
be fully read-only.

To invert an inversion list, one simply unshifts a 0 to the front of it
if one is not there, and shifts off the 0 if it does have one.

The current data structure reserves an element at the beginning of each
inversion list that is either 0 or 1.  If 0, it means the inversion list
begins there; if 1, it means the inversion list starts at the next
element.  Inverting involves flipping this bit.

This commit changes the structure so that there is an additional element
just after the element that flips.  This new element is always 0, and
the flipping element now says whether the inversion list begins at the
constant 0 element, or the one after that.

Doing this allows the flipping element to be separated in later commits
from the body of the inversion list, which will always begin with the
constant 0 element.  That means that the body of the inversion list can
be const.
Reinstate "regcomp.c: Make inversion lists SVt_PVLV"

This reverts commit ac7a6f5849b5fd57b2e837f4ddbc18c992610e9c which
reverted 2c3365de8c1168f115576a4976d067e3b911c490, thus reinstating the
latter commit.  It turns out that the error being chased down was not
due to this commit.

This commit additionally changes some now-obsolete wording in a pod.
This change was not in 2c3365de8c1168f115576a4976d067e3b911c490.

The original message for commit 2c3365de8c1168f115576a4976d067e3b911c490
was:

This is the 2nd step in separating the inversion list body from header.
This commit gives inversion lists the header from a SVt_PVLV, and
repurposes one of its fields into being the length of the inversion
list.

This is a temporary measure, in case binary compatibility is an issue.
Future commits will create a new SV type just for inversion lists.  This
SV type was chosen because it has a sufficient number of fields to
accommodate all the header fields from inversion lists.
Reinstate "regcomp.c: Remove unused data structure field"

This reverts commit c7995b20ccfbb4248b23aeae9fd1eab838852fcc, which
reverted 4b98096221966ea01c046f4f61b2dc4f60b534b9, thus reinstating
the latter commit.  It turns out that the error being chased down was
not due to this commit.

Its original message was:

This removes a field that is set in the inversion list data structure
and examined just once.  And that sole examiner is the function that
calls the function that does the set.  In other words X calls Y passing
it data D.  Y puts D into a structure.  Upon return from Y, X looks for
D in the structure.  No one else looks at D.  X might just as well have
looked at D directly, without involving Y, and without the structure
needing a space for D.
Reinstate "regcomp.c: Move 2 hdr inversion fields to SV hdr"

This reverts commit 67434bafe4f2406e7c92e69013aecd446c896a9a, which
reverted 4fdeca7844470c929f35857f49078db1fd124dbc, thus reinstating the
latter commit.  It turns out that the error being chased down was not
due to this commit.

Its original message was:

This commit continues the process of separating the header area of
inversion lists from the body.  2 more fields are moved out of the
header portion of the inversion list, and into the header portion of the
SV that contains it.
Reinstate  "regcomp.c: Change, variable, fcn name"

This reverts commit 0b58015e05b2ab93b080b7c49a70bf82435363c0, which
reverted 875c4e2c5193b5245da578b222e9c93aad31d93b, thus reinstating the
latter commit.  It turns out that the error being chased down was not
due to this commit.

Its original message was (slightly revised for clarity):

These have always been slightly misnamed, but a recent commit made them
more so.  The old name contained "zero", but now there is a new element
which always has zero.  The renamed element indicates whether the
inversion list is offset, that is if the beginning is the zero element,
or if the beginning is the next element beyond the zero element.
Reinstate "regcomp.c: Move inversion list hdr field to SV hdr"

This reverts commit 2eb2feb9f4a226d0fe0fd3d66e2ce341296f0072, which
reverted d913fb457b732da4c31d0d1b8c085989a7ecd12d, thus reinstating the
latter commit.  It turns out that the error being chased down was not
due to this commit.

Its original message was:

This moves the final field that can vary from the inversion list data
structure into the header of the SV that contains it.  With this commit,
the body of an inversion list is now const.

The field is converted to a U8, to correspond with the header field in
the SV type that we currently use to hold inversion lists.
Reinstate "regcomp.c: Make C-array inversion lists const"

This reverts commit 18505f093a44607b687ae5fe644872f835f66313, which
reverted 241136e0ed70738cccd6c4b20ce12b26231f30e5, thus reinstating the
latter commit.  It turns out that the error being chased down was not
due to this commit.

Its original message was:

The inversion lists that are compiled into a C header are now const.
Reinstate "regcomp.c: Move some #defines to only file that uses them"

This reverts commit 247f9b19318882fd9a52fe49aa31fc8d3d3db4f7, which
reverted 05944450e0fc82eb8fc1fb5a4bf63f23785262a0, thus reinstating the
latter commit.  It turns out that the error being chased down was not
due to this commit.

Its original message was:

These were used in a header file to provide synchronization between
files.  However, the only other file that would need them is a .pl file
which doesn't have access to them.  So simplify things so that the
variables are either removed entirely if only used in a single place, or
are #defined in the area where they are used
Reinstate "Create SVt_INVLIST"

This reverts commit 49cf1d6641a6dfd301302f616e4f25595dcc65d4, which
reverted e045dbedc7da04e20cc8cfccec8a2e3ccc62cc8b, thus reinstating the
latter commit.  It turns out that the error being chased down was not
due to this commit.

Its original message was:

This reshuffles the svtype enum to remove the dummy slot created in a
previous commit, and add the new SVt_INVLIST type in its proper order.
It still is unused, but since it is an extension of SVt_PV, it must be
greater than that type's enum value.  Since it can't be upgraded to any
other PV type, it comes right after SVt_PV.

 Affected tables in the core are updated.
Reinstate "Use new Svt_INVLIST for inversion lists."

This reverts commit 2e0b8fbeab3502bee36f25825c3cdd0d075c4fd3, which
reverted e0ce103ae532f9576f54a5938a24d1ee98dfb928, thus reinstating the
latter commit.  It turns out that the error being chased down was not
due to this commit.

Its original message was:

This converts inversion lists to use their own scalar type.
regcomp.c:  Only White-space, comments, name of variable change
regcomp.c: Remove now useless initialization

This code was designed to cause a segfault by initializing an offset
into an address to a very large value.  But now, it is stored as a bool,
so there is not point in doing this.  We also remove an assert that it
is a bool, because it always has to be.
regcomp.c: Reorder a couple of function calls

A future commit with require that the offset be known before the length
gets set.  This prepares for that.
Fix off-by-one error in inversion lists.

The first commit of this topic branch added a dummy 0 element to the end
of certain inversion lists to work around an off-by-one error.  This
commit makes the necessary changes to stop that error, and to remove
the dummy element.  SvCUR() and invlist_len() now are kept in sync.
regcomp.c: Rmv now redundant setting

SvCUR is now properly set in invlist_set_len().  There is no need to do
it outside that function.
Add parameter to internal function

The function invlist_set_len() has to be called after the offset header
field in an inversion list has been set.  To make sure that future
maintainers don't forget to do this, add the parameter for the 'offset'
to its call, so it can't be called without having this value.
later