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
While there exists no code that would/could cause this to fail, officially we do not guarantee that you will ever get exactly the capacity you request. In theory we would like to support the allocator coming back with more than you asked for.