Provide a signature! macro that constructs a const singature

[zeenix]

This was discussed before but at the time, we didn't have the means to do it. Now it should quite easy, with the parsed signature type. In fact, the Type macro is already doing it for its signature attribute. This would be mostly about moving that logic out into a separate macro. The folks can do things like:

let singature = signature!("a{sv}");
const SIGNATURE: Signature = signature!("a{sv}");

with the string checked at compile-time.