As the call to `symbol_is_used` does not depend
on the production, it is more efficient to call it
only once outside the loop over productions.
I'm not sure if `rustc` is able to do this optimization
on its own (it would need to know that the function
is pure, which sounds difficult in general).
* 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
