global match data and context

[Reini Urban]

See PR #18 for the data part, #6 for the context parts.
There are two independent global contexts: compile and match.

#18 regressed in one t/perl/regexp.t test. 1840 with an uninitialized stack element st.reg_sv.

# check we have the right utf8ness as we switch back and forth between
# patterns
^(\x{100}|a)(??{ qr/.?\xF7/d})	a_\xF7	y	-	-

sometimes this even gives an regexp memory corruption error in the #18 branch match-data-comp-alloc, sometimes also corrupted regexp pointers. Wrong restauration of st.reg_sv after the nested codeblock fallback.
asan reports a heap-buffer-overflow in S_regmatch.

[Reini Urban]

Merged Avar's match part #18 with 0.14,
accepting the single regression and memory corruption.