* 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
`cargo xtask test test_something` currently constructs `cargo test ''
test_something`, which errors out for `cargo test`. This fix removes
the `test_flags` string if it's empty.
