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
Actually, it occurs to me: can't you make this test capacity agnostic by just doing push_back()x2, push_front()x2? Or are you testing some particular edge cases?