RFC: Array pattern adjustments

Change array/slice patterns in the following ways:

- Make them only match on arrays (`[T, ..n]` and `[T]`), not slices (#478);
- Make subslice matching yield a value of type `[T, ..n]` or `[T]`, not `&[T]` or `&mut [T]`;
- Allow multiple mutable references to be made to different parts of the same array or slice in array patterns (resolving rust-lang/rust#8636).