* 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
Instead of having users declare the extern function themselves, they can
pass in the language to `Language.create` in the zig bindings. If they
really want, they can always opt into the `extern fn tree_sitter_LANG()
*const ts.Language` approach.