Add RingBuf::as_slices as per collections reform v2.

See: https://github.com/rust-lang/rfcs/pull/509

Not sure if this is allowed to land before the RFC. Either way,
it's here for review.

r? @Gankro
cc: @bfops
Doesn't work for me. Weird error, too:

```
<std macros>:6:23: 6:50 error: the trait `core::kinds::Sized` is not implemented for the type `[int]`
```