Compile-time checking for %$obj{"key"} under ‘use fields’

This single line of code also gives us shared hash key optimisa-
tions for free.
Increase $constant::VERSION to 1.29
constant.pm: Consistent spaces after dots in pod

It was inconsistent throughout.
sv.c apidocs: SV_NOSTEAL, not NOSTEAL

NOSTEAL is nonexistent.  While abbreviating constants may be fine for
comments, using such forms in API documentation may be less than
helpful.
Don’t skip t/re/reg_eval_scope.t under miniperl

re.pm doesn’t load its C code under miniperl, and it’s not needed anyway
here, since we are just setting hints.