Add a rustfmt.toml file matching rust-lang/rust

Duplicate the settings from rust-lang/rust to this repository. This is
mostly for consistency, but `use_small_heuristics = "Max"` does make a
large difference with lookup tables.

Also apply the needed CI changes to run nightly rustfmt.

library/compiler-builtins/libm/.github/workflows/main.yml

@@ -41,7 +41,7 @@ jobs:
     steps:
     - uses: actions/checkout@master
     - name: Install Rust
-      run: rustup update stable && rustup default stable && rustup component add rustfmt
+      run: rustup update nightly && rustup default nightly && rustup component add rustfmt
     - run: cargo fmt -- --check
 
   wasm:

library/compiler-builtins/libm/.rustfmt.toml

@@ -0,0 +1,5 @@
+# This matches rustc
+style_edition = "2024"
+use_small_heuristics = "Max"
+group_imports = "StdExternalCrate"
+imports_granularity = "Module"