Move bulk of pp_regcomp() into re_op_compile()

Move bulk of pp_regcomp() into re_op_compile()

When called, pp_regcomp() is presented with a list of SVs on the stack.
Previously, it would perform (amongst other things):
  * overloading those SVs;
  * concatenating them;
  * detection of bare /$qr/;
  * detection of unchanged pattern;
optionally followed by a call to the built-in or an external regexp
compiler.

Since we want to avoid premature concatenation (so that we can handle
/$runtime(?{...})/), move all these activities from pp_regcomp() into
re_op_compile().

This makes re_op_compile() a bit cumbersome, with a large arg list,
but I haven't found any way of only moving only a subset of the above.

Note that a side-effect of this is that qr-overloading now works for all
regex compilations, not just those reached via pp_regcomp(); in particular
this now invokes the qr method rather than the "" method if available:
/(??{ $overloaded_object })/