add regmatch_eval_state struct

add regmatch_eval_state struct

Replace several PL_reg* vars with a new struct. This is part of the
goal of removing all global regex state.

These particular vars are used in the case of a regex with (?{}) code
blocks. In this case, when the code in a block is called, various bits of
state (such as $1, pos($_)) are temporarily set up, even though the match
has not yet completed.

This involves updating the current PL_curpm to point to a fake PMOP which
points to the regex currently being executed. That regex has all its
current fields that are associated with captures (such as subbeg)
temporarily saved and overwritten with the current partial match results.
Similarly, $_ is temporarily aliased to the current match string, and any
old pos() position is saved. This saving was formerly done to the various
PL_reg* vars.

When the regex has finished executing (or if the code block croaks),
its fields are restored to the original values. Since this can happen in a
croak, it may be done using SAVEDESTRUCTOR_X() on the save stack. This
precludes just moving the PL_reg* vars into the regmatch_info struct,
since that is just allocated as a local var in regexec_flags(), and would
have already been abandoned and possibly overwritten after the croak and
longjmp, but before the SAVEDESTRUCTOR_X() action is taken.

So instead we put all the vars into new struct, and malloc that on entry to
the regex engine when we know we need to copy the various fields.
We save a pointer to that in the regmatch_info struct, as well as passing
it to SAVEDESTRUCTOR_X(). The destructor may get called up to twice in the
non-croak case: first it's called explicitly at the end of regexec_flags(),
which restores subbeg etc; then again from the savestack, which just
free()s the struct. In the croak case, it's called just once, and does
both the restoring and the freeing.

The vars / PL_reg_state fields this commit eliminates are:

    re_state_eval_setup_done
    PL_reg_oldsaved
    PL_reg_oldsavedlen
    PL_reg_oldsavedoffset
    PL_reg_oldsavedcoffset
    PL_reg_magic
    PL_reg_oldpos
    PL_nrs
    PL_reg_oldcurpm