refactor: make compilers testable without backends where possible

[cpcloud]

Thinking about the new compilers, and being a bit frustrated by having non-backend-needing tests, like compiler tests fail after merge, I wonder if we should split out the compilers into their own subpackage so that the compilers can be tested without being run as part of the backend test suite. This would hopefully allow us to write tests that run quickly and don't require a backend, but still hit everything up to execution.

[jcrist]

Would this include the snapshot tests (the primary culprits I run into regarding this)? If so, I'm a +100 on making this possible.

[cpcloud]

Definitely would like to include snapshots in this.

[cpcloud]

Also open to approaches that enable this without the churn, just mentioned the one in the issue description as a possible one.

[gforsyth]

DONE