Reorganize rust crates into a flat crates directory, simplify some CI steps (#4496)

* 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

cli/src/templates/gitignore

@@ -1,50 +0,0 @@
-# Rust artifacts
-target/
-Cargo.lock
-
-# Node artifacts
-build/
-prebuilds/
-node_modules/
-package-lock.json
-
-# Swift artifacts
-.build/
-Package.resolved
-
-# Go artifacts
-_obj/
-
-# Python artifacts
-.venv/
-dist/
-*.egg-info
-*.whl
-
-# C artifacts
-*.a
-*.so
-*.so.*
-*.dylib
-*.dll
-*.pc
-*.exp
-*.lib
-
-# Zig artifacts
-.zig-cache/
-zig-cache/
-zig-out/
-
-# Example dirs
-/examples/*/
-
-# Grammar volatiles
-*.wasm
-*.obj
-*.o
-
-# Archives
-*.tar.gz
-*.tgz
-*.zip