Cargo.toml: add `license` field

Cargo's docs note that "If a package is using a nonstandard license, then the license-file field may be specified in lieu of the license field," https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields

Since this crate is using the standard MIT license, it's preferred to have a license = ... field instead. (cargo build complains if both exist)