Set default license + Add Substrate's `rustfmt` config (#69)
* Add license * Add rustfmt config * Add `fmt` CI job * Add `FILE_HEADER` to files * Apply `cargo fmt` * Revert "Apply `cargo fmt`" This reverts commit 1bfb7a289ff093a9cc1e22318e9c96f5aa9cbfb0. * Copy `rustfmt.toml` from Substrate over * Apply `cargo fmt` * Add license check * Fix stage name * Use nightly for `rustfmt`