Instead of returning an undocumented boolean flag, use a
core::ops::ControlFlow object. At the expense of being a bit more
verbose, this is a type that should be self-explanatory in the context
of a callback, as an indication of whether to continue processing or
stop.
* 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
