Add `fixed_result_into` to prevent copy

[bwesterb]

Rust does not (yet) implement named return value optimisation, so fixed_result forces a copy.

How would you feel about adding a

fn fixed_result_into(mut self, out : &mut GenericArray<u8, Self::OutputSize>)

function to the FixedOutput trait so that the copy can be avoided. I'm happy to create a PR.

(Background: I'm implementing the hash-based signature XMSSMT in rust, where lots of hashes are computed on on very short inputs. These copies have a significant overhead.)

[tarcieri]

It seems like with const generics on the horizon there should (possibly) be a way to do this in terms of FromIterator soon? e.g.

https://internals.rust-lang.org/t/collecting-iterators-into-arrays/10330

[newpavlov]

I was thinking about doing the following re-design in the next digest version:

pub trait FixedOutput {
    type OutputSize: ArrayLength<u8>;

    /// Private method not intended to be used directly
    fn _fixed_result_core(&mut self, out: &mut GenericArray<u8, Self::OutputSize>, reset: bool);

    #[inline]
    fn fixed_result(self) -> GenericArray<u8, Self::OutputSize> {
        let mut buf = Default::default();
        self._fixed_result_core(&mut buf, false);
        buf
    }

    #[inline]
    fn fixed_result_reset(&mut self) -> GenericArray<u8, Self::OutputSize> { .. }
    fn fixed_result_into(self, out: &mut GenericArray<u8, Self::OutputSize>) { .. }
    fn fixed_result_into_reset(&mut self, out: &mut GenericArray<u8, Self::OutputSize>) { .. }
}

I think #[inline] should help with needless copies.