add op_comp field to regexp_engine API

add op_comp field to regexp_engine API

Perl's internal function for compiling regexes that knows about code
blocks, Perl_re_op_compile, isn't part of the engine API. However, the
way that regcomp.c is dual-lifed as ext/re/re_comp.c with debugging
compiled in, means that Perl_re_op_compile is also compiled as
my_re_op_compile. These days days the mechanism to choose whether to call
the main functions or the debugging my_* functions when 'use re debug' is
in scope, is the re engine API jump table. Ergo, to ensure that
my_re_op_compile gets called appropriately, this method needs adding to
the jump table.

So, I've added it, but documented as 'for perl internal use only, set to
null in your engine'.

I've also updated current_re_engine() to always return a pointer to a jump
table, even if we're using the internal engine (formerly it returned
null). This then allows us to use the simple condition (eng->op_comp)
to determine whether the current engine supports code blocks.