librustc: Feature gate subslice matching in non-tail positions.

This breaks code that uses the `..xs` form anywhere but at the end of a
slice. For example:

    match foo {
        [ 1, ..xs, 2 ]
        [ ..xs, 1, 2 ]
    }

Add the `#![feature(advanced_slice_patterns)]` gate to reenable the
syntax.

RFC #54.

Closes #16951.

[breaking-change]

r? @alexcrichton
Bad reformat?