* Move all rust crates (except lib) into crates dir, w/o nesting
* Remove stale path from .gitattributes
* Rename lib.rs files for easier navigation
* Rename mod.rs file for easier navigation
* Fix emscripten-version path
* Fix fixtures dir paths
* Use the default rustfmt settings
* Don't use nightly on CI
Introduces a new function that takes in a supertype symbol and returns
all associated subtypes. Can be used by query.c to give better errors
for invalid subtypes, as well as downstream applications like the query
LSP to give better diagnostics.
