add lex_re_reparsing boolean to yy_parser struct

add lex_re_reparsing boolean to yy_parser struct

When re-parsing a pattern for run-time (?{}) code blocks,
we end up with the EVAL_RE_REPARSING flag set in PL_in_eval.
Currently we clear this flag as soon as scan_str() returns, to ensure that
it's not set if we happen to parse further patterns (e.g. within the
(?{ ... }) code itself.

However, a soon-to-be-applied bugfix requires us to know the reparsing
state beyond this point. To solve this, we add a new boolean flag to the
parser struct, which is set from PL_in_eval in S_sublex_push() (with the
old value being saved). This allows us to have the flag around for the
entire pattern string parsing phase, without it affecting nested pattern
compilation.