**Problem:** When encountering an invalid symbol at the beginning of the
file, the rust bindings attempt to index the character at position -1 of
the query source, which leads to an overflow and thus invalid character
index which causes a panic.
**Solution:** Bounds check the offset before performing the subtraction.
* 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
