Optimize static methods to subs

[rurban]

Was CM-885, part of the GH #23 inliner.

See http://blogs.perl.org/users/rurban/2011/06/how-perl-calls-subs-and-methods.html
It is better to do it with the inline peep check, and not in ck_subr. (Now changed back to ck_subr)
So if any stash already contains the method, use it. No other stash can interfer with that during run-time method-dispatch.

But we cannot use the $obj->method type info, because $obj may be a subtype
of the declared type, which may have a different method.

Add several smaller optims:

• convert pushmark-gv(packagename)-args-method_named(GV)-entersub
  to pushmark-gv(packagename)-args-gv(GV)-entersub

• use the faster cvref not the gv. the biggest problem was that destroying &main::BEGIN cleared the attached GV (missing refcount bump):
  Not when a static method is allowed to be redefined. perlcc -O3 may disallow this for perf. reasons.
  t/op/symbolcache.t wants to redefine a sub at run-time and this can only be caught with the GV, not the old cvref.

• convert entersub to enterxssub for XS methods.

• detect class subs called as methods at compile-time already.

• using a gv ALIAS e.g. *Carp::Import{CV} pointing to &Exporter::import is forbidden. This needs to be resolved by method_named.

This should be merged earlier than the inliner, because the stable parts are needed for other branches to go on. op_clone() is needed to clone roles #311.

[rurban]

• cvt method sub -> xs done at run-time since v5.27.1c (596c328) in METHOD_CHECK_CACHE. this is safe even when a stash is invalidated.

[rurban]

• conversion of method_named -> gv done with 4c9c0ae but only for non-threaded perls. still an unknown corruption with that gv on the pad at run-time.
  also done: skip if gv alias.

not yet done:

• class/subclasses with typed lhs objects
• cvref (probably not possible, see symbolcache)

[rurban]

Threaded perls fixed with a386e87
It was pad_alloc flag problem. With the GV as SVf_READONLY it will be not reused for another TARG.