speedup for SUPER::method() calls.

In ck_method:
Scan for '/::. If found SUPER::, create OP_METHOD_SUPER op
with precomputed hash value for method name.

In B::*, added support for method_super

In pp_hot.c, pp_method_*:
S_method_common removed, code related to getting stash is
moved to S_opmethod_stash, other code is moved to
pp_method_* functions.

As a result, SUPER::func() calls speeded up by 50%.
Opcode.pm: wrap long verbatim pod line