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
This is true. When that happens, this test will have to be updated.

Until then, I still don't know of a better way to write this test.