hash access

[Leopold Toetsch]

We have keyed_int shortcuts to get/set items on array like aggregates.
Is there a reason, that we not have keyed_str to access hash elements by 
STRING directly.
Currently a hash access inside parrot needs a key_new_string() which 
contructs a new Key PMC, where the string then gets extracted again.

This would add ~15 vtable entries: {get,set}_<type>_keyed_str, type, 
exists, defined, delete. Ah yes, and some ops variations ;-)

Comments welcome
leo