add Perl_re_op_compile function

add Perl_re_op_compile function

Make Perl_re_compile() a thin wrapper around a new function,
Perl_re_op_compile(). This function can take either a string pattern or a
list of ops. Then make pmruntime() pass a list of ops directly to it, rather
concatenating all the consts into a single string and passing the const to
Perl_re_compile(). For now, Perl_re_op_compile just does the same: if its
passed an op tree rather than an SV, then it just concats the consts.

So this is is just the next step towards eventually allowing the regex
engine to use the ops directly.