[PMC] Sped up HashIterator's shift_string VTABLE.
[PMC] Sped up HashIterator's shift_string VTABLE.

Removing the delegation to shift_pmc() avoids creating a temporary PMC, which
speeds up the Rakudo startup benchmark by ~6.23%, and should help Rakudo run
time and anything else which performs Class introspection.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@47816 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot