regcomp.c: Centralize assignment

It's better to do something in one common place than two.  This properly
initializes the regex opcode for the synthetic start class when it is
created, rather than at the end where the code has to be repeated to get
all instances.
regcomp.c: Remove redundant matching possibilities

The flag ANYOF_UNICODE_ALL is for performance.  It is set when the
inversion list for the ANYOF node includes every code point above
Latin1, and avoids runtime searching through the list.  We don't need
both, as the flag being set short-circuits even looking at the other
list.  By removing the code points from the list, we perhaps will get
rid of the list entirely, thus saving some operations, or will shorten
it so that later binary searches run faster.
regcomp.c: Extract code into separate function

This is in preparation for it to be called from more than one place, in
a future commit.
Add inversion list for U+80 - U+FF

This is the upper half of the Latin1 range.  This simplifies some code
very slightly, but will be of use in future commits.
regcomp.c: Move a #define, add a similar one

Future commits will use this #define (and the new one) earlier in the
file than currently defined.
Add regnode struct for synthetic start class

As part of extending the regular expression optimizer to properly handle
above Latin1 code points, I need an inversion list to contain which code
points the synthetic start class matches.

The synthetic start class currently is the same as an ANYOF node with
locale, which uses the struct of a regular ANYOF node, pluse some extra
fields at the end.  This commit creates a new struct which just The arg1 field of
regcomp.c: Use PL_sv_undef instead of NULL in an AV

The NULL gets turned into an SVt_NULL anyway.  This array is read only
by S_core_regclass_swash() in regexec.c.  That uses an SvROK, so it
doesn't have to change.

This commit also beefs up the comments around this operation
regcomp.c: Extract code into separate function

A future commit will use this functionality from another place.  For
now, just cut and paste, and do the minimal ancillary work to get it to
compile and pass.
regcomp.c: Rename formal param; comments

The previous commit just extracted out code into a function.  This
commit renames a parameter for clarity, combines two parameters to make
the interface cleaner, and adds and moves comments around.
regcomp.h, sv.c, utf8.c: Comment nits

Remove obsolete comment, typos in others, plus reflow one block to fit
into 79 columns
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