Cranelift: state which crates are public and semver-compatible

[cfallin]

The discussion around #3420 has made clear that we should, at a minimum, specify which crates we consider to be "internal" and which we consider to be "public" for the purposes of semver compatibility. This issue, to be clear, does not require (for now) any sort of process or stability policy beyond a list of crates; just that we should say "if you use crates X, Y, and Z, then we will follow semver".

I propose that we declare at least the following as such:

• cranelift-codegen
• cranelift-frontend
• cranelift-wasm
• cranelift-object
• cranelift-jit
• cranelift-native
• data structures foundations: cranelift-entity / cranelift-bforest

Any others I've missed?