Add APIs for coroutines equivalent to those for closures

[momvart]

Coroutines and closures have quite similar type structures. It makes sense to provide similar APIs for coroutines as those that exist for closures, like:

• Context::closure_sig
• Context::resolve_closure
• Coverage in Context::fn_sig

[oli-obk]

I don't know if we'll be able to promise that we'll be able to refactor all of these APIs in the background without breaking the SMIR API. They change really rarely, so maybe that's not an issue

[momvart]

I don't know if we'll be able to promise that we'll be able to refactor all of these APIs in the background without breaking the SMIR API.

True. Before anything, it's "stable" MIR.

They change really rarely, so maybe that's not an issue

Yeah. Also, we currently have CoroutineDef, and some APIs for it like getting the signature seem to be quite static, even if the background changes in Context.