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/npm/cli.js

@@ -1,12 +0,0 @@
-#!/usr/bin/env node
-
-const path = require('path');
-const spawn = require("child_process").spawn;
-const executable = process.platform === 'win32'
-  ? 'tree-sitter.exe'
-  : 'tree-sitter';
-spawn(
-  path.join(__dirname, executable),
-  process.argv.slice(2),
-  {stdio: 'inherit'}
-).on('close', process.exit)