The loader package's `ensure_wasi_sdk_exists` private method checks for
the wasi-sdk, fetching it if it can't be found. This logic was
re-implemented in xtask for `build-wasm-stdlib`, but without the
fetching functionality. We can have nice things in xtask too! Rather
than make this function a public member of `tree-sitter-loader`, we
just re-implement and leave a nice comment asking people to keep the
two in sync.
* 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
