Add accessor to get the pad index for a lexical variable/declaration.
Add more types used in Emit.pm

ANY can only be used in the requested type for an expression, SV
and GV can be used both as requested type and returned type and map
to the native Perl values.

There are no AV and HV for now (it's better to wait for an actual use
case, it might be that Array/Hash types need some refactroing anyway).
Small refactoring.
Refactor the code to assign a JITted value to an SV.
Slightly better context handling for optrees and JITted return value.

Should handle scalar, list and void context (under the assumption
that in list context the values are returned on the stack by the
JITted operation).

Caller-determined context is not implemented (and dies for now) but
it will need to be implemented on an op-by-op basis.