This commit speeds up class method calls when class name is constant.

I.e.
MyClass->method()
and
MyClass->$dynamic_method()

By about 25%.

It was done by saving class name (as shared COW string) in SVOP
and later checking it in method_common().

If it was set, then it fetches stash via gv_stashsv using precomputed
hash value instead of falling into a bunch of conditions and fetching
stash without hash value.