Tell rustfmt about the edition we use (#96)

My editor (Emacs) likes to format individual files by sending them to
`rustfmt`. This fails without a configuration file to set the edition
since `rustfmt` (when executed alone, not via `cargo fmt`) defaults to
the good old Rust 2015 edition.

rustfmt.toml

@@ -0,0 +1 @@
+edition = "2021"