cache compilations to speed up startup

[ryanatkn]

This PR introduces more sophisticated caching to avoid unnecessary work on startup. Previously, the Filer re-compiled everything on startup and then compared results to what was on disk, which is usually wasted work. The Filer now stores a JSON metadata file corresponding to each source file (including externals) with the minimum amount of information needed to rehydrate compilations on startup. Right now, this metadata includes 1) a hash of the source contents, 2) a list of the compiled files' paths, which correspond to built files on disk, and 3) their encoding, like utf8 or null.

By combining the metadata with the built files on disk, we can avoid re-compiling files on startup. This cut the gro dev startup time in half for Gro itself, and for frontend projects that use a lot of Svelte, this could improve startup time by orders of magnitude, depending on the size of the project.

There remain corner cases that require the developer to manually clear the cache, like updating dependencies or changing compilation options that aren't handled by the Filer. We can mitigate some of these problems down the road - for now, if something looks off, gro clean should be the go-to fix.