replace illegal empty L<> with C<< LE<lt>E<gt> >>

amusingly, the "reject empty L<>" check was rejecting the documentation
of the "reject empty L<>" check!
start expecting the new =item-changed-type warning
we get back warning if we turn them on

Old version had set warnings to 200, and it was lost in conversion.
This turns it back on!
avoid hash ordering bug in tests!
eliminate the utterly broken "unescaped <" warning

It's not going to work, because the content that is in the paragraph
has already been E<>-decoded.  *Everything* is unescaped.  This needs
to be caught during parsing, I think.  I don't think that the para_end
event can have enough information.

I think we're okay without this warning for now, but it would be nice
to put it back.  In the meantime, rip it out.  The change to
podchkinter is to replace that "low severity warning" with another
one, since the test is for the behavior of -warnings, not the <
warning itself.
remove weird added-later remark about "unescaped <"

I think it was added as a later misinterpretation of the way in
which the warning had been broken.
remove mention of unescaped <

In fact, I think this test is not useful.  We already have
a test for unknown formatting codes.  Nothing else should
be ambiguous.
item type mismatch was moved to Pod::Simple
use the same warning/error level as in Pod::Simple
rely on =over w/o closing =back being in Pod::Simple
eliminate the list stack, thus eliminating use of Pod::List
expect new L<> warnings from Pod::Simple
stop passing on Pod::Hyperlink warnings as warnings

This is part of the work to drop Pod::Hyperlink entirely.  Note that
this also drops a warning for L</I/O Foo>, which is no longer
ambiguous, and was already a buggy check.

A warning for this may be re-introduced via Pod::Simple in the
future, but probably not until Pod::Simple's L<> processing is
made less complex.
reimplement unresolved internal hyperlink check

this now uses Pod::Simple facilities rather than Pod::ParseUtils,
eliminating any reliance on Pod::Parser