Begining the work on weak references in Rc: redesign the protocol, define the weak type, and update the code for rc.

This commit makes typed_val and typed_instruction_ty a definition. Together
with the previous commit on notations for contexts, this seems to fix all
notations issues.

Instead, I have added subscripts to the inclusion notations. This makes pretty
printing much more reliable, and generally even shortens things, as no scope
annotations are needed.