How-to merge Context and CallSignature.

1. Merge those two PMC into single CallContext (done).
2. Parrot_pcc_build_sigobject updated to create CallContext instead of CallSignature (done).
3. Parrot_pcc_invoke_from_sigobject, op invoke, op invokecc should push CallContext created in step 2.
4. Sub.invoke shouldn't push new CallContext, just allocate registers in CURRENT_CONTEXT.
5. op tailcall should just merge two CallContext (parent and current) (same as in trunk).

Steps 3-5 are questionable and require review by allison.