[Rust] Support #![] module level attributes

[rrichardson]

When attempting to put
#![allow(clippy)] at the top of a file, it gets placed below any imports created by register.

It'd be nice to specify, perhaps in the Rust config, any top-of-module lines that must be present.

The workaround is to just insert such things first at the top of a file, but this sort of breaks the abstraction.

[udoprog]

Hah, for sure! Maybe the import location should be specified explicitly or something like it.

[millergarym]

Hah, for sure! Maybe the import location should be specified explicitly or something like it.

Another option would be for an explicit header similar to rust::import (lang::import) but for headers.
This feature could also be used for comments.

e.g.

let header = rust::header("/* generated code using the assume genco library */");
let clippy = rust::header("#![allow(clippy)]");
let tokens = quote{ $(register( (header, clippy) );
...
};