Closures should accept & for arguments

[Pauan]

Motivation

If a Closure accepts & by argument it can use the faster stack-allocation rather than the slower heap-allocation.

Basically, this should be possible:

let f = move |e: &Foo| { ... };
let x = Closure::wrap(Box::new(f) as Box<FnMut(&Foo)>);

Additional Context

rustwasm/gloo#30 (comment)

[alexcrichton]

Out of curiosity, since I think that a lot of code duplication would be required here is there perhaps one or two signatures which would hit 90% of use cases? For example would FnMut(&T) be enough to cover basically everything?

[Pauan]

@alexcrichton It would certainly cover our particular use case (of event listeners). Though we would need FnOnce(&T) as well. So that's 2 signatures.

[alexcrichton]

Ok I've posted an initial idea of this in #1417