Infer type in irrefutable slice patterns with fixed length as array

[b-naber]

Fixes #76342

In irrefutable slice patterns with a fixed length, we can infer the type as an array type. We now choose to prefer some implementations over others, e.g. in:

struct Zeroes;

const ARR: [usize; 2] = [0; 2];
const ARR2: [usize; 2] = [2; 2];

impl Into<&'static [usize; 2]> for Zeroes {
    fn into(self) -> &'static [usize; 2] {
        &ARR
    }
}

impl Into<&'static [usize]> for Zeroes {
    fn into(self) -> &'static [usize] {
        &ARR2
    }
}

fn main() {
    let &[a, b] = Zeroes.into();
}

We now prefer the impl candidate impl Into<&'static [usize; 2]> for Zeroes, it's not entirely clear to me that this is correct, but given that the slice impl would require a type annotation anyway, this doesn't seem unreasonable.

r? @lcnr