On-disk module cache

[Stebalien]

At the moment, we have no support for on-disk module caching. Wasmtime has built-in support for this but:

1. It's global (system wide).
2. It compresses modules. We end up spending a lot of time decompressing modules at runtime.

Now that we have an in-memory cache (inside the engine), we should add support for persisting this cache to disk. The first use-case will be speeding up conformance tests in CI by caching built modules in github actions.