witx: Add trait implementations to Id

[cratelyn]

While working on bytecodealliance/wasmtime#1793, I noticed that Id cannot be compared against strings, or referenced as a string slice. While as_str can be used, this often means hanging onto a temporary value, i.e. let id_str: &str = id.as_str().

Because Id is a newtype wrapper around a single String, it felt like these would be both useful & safe to expose a few more trait implementations, delegating to the underlying String value.

After applying these changes, this code will compile:

use witx::Id;

fn main() {
    let id = Id::new("hello");

    // Compare to string slices, as the lhs.
    let _ = id == "hello";

    // Compare to string slices, as the rhs.
    let _ = "hello" == id;

    // Call `Into::into` and `From::from` on string slices.
    let _: Id = "call into on a string".into();
    let _ = Id::from("call from, on a string");
}