No type inference for slice patterns

[jplatte]

Minimal reproduction:

let xs: &[f64] = &[0.0];
match xs {
    &[x] => {
        x; // {unknown}
    }
    _ => {}
}

[flodiebold]

We already have a Pat::SlicePat variant, so the first step would be implementing lowering from AST to hir_def::expr::Pat here. After that, we need to implement the actual inference here; basically, check whether the expected type is a slice, then recursively call infer_pat on the parts (for the prefix and suffix patterns, the expected type should be the element type; for the rest pattern, it'll be the slice type itself.) It's a bit similar to tuple patterns, which are implemented a bit further up:
https://github.com/rust-analyzer/rust-analyzer/blob/5397f05bfe7f3b18229a65040c6685e762b2f9a3/crates/ra_hir_ty/src/infer/pat.rs#L115-L129

[jplatte]

Forgot to remove the 'will fix' from the PR. Can't reopen the issue though.