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
You can do basically what I did in the insert test. Check what you get back, and use ranges or whatever to produce as much as you need to get the behaviour you want. 

I don't *suuuuper* care, but it would be nice to be robust here (also easy to "ratchet" the test later, fwiw).