(POC of) multi-crate MIR execution

[jberthold]

Exploration of multi-crate (cargo generated) multi-smir compilation showed that we need to extend the #init function to consume multiple Pgms instead of a single one, as well as modifications to how functions are looked up and loaded (we need a crate context).

Based on the brainstorming done (video here, AI notes available), make an experimental branch for small multi-crate testing.
We will keep this branch alive but not merge it to avoid complications while working on the core semantics.

[dkcumming]

Here is the example that we were working on with crate boundaries.
example2.zip