RFC: overloaded slice notation

This RFC adds *overloaded slice notation*:

- `foo[]` for `foo.as_slice()`
- `foo[n..m]` for `foo.slice(n, m)`
- `foo[n..]` for `foo.slice_from(n)`
- `foo[..m]` for `foo.slice_to(m)`
- `mut` variants of all the above

via two new traits, `Slice` and `SliceMut`.

[Rendered](https://github.com/aturon/rfcs/blob/slice-notation/active/0000-slice-notation.md)
I'm not 100% sure that e.g. `foo(some_string[]);` is better from a first impressions perspective, it's rather cryptic/hard-to-google (sigil like?) syntax, unlike `foo(some_string.as_slice())`.