[collections] Enhancement: refactor RingBuffer to operate more like a pipe

[kyleholohan]

This PR includes the following enhancements:

• Refactors the RingBuffer class to operate more like a pipe: once instantiated, it may be opened to produce a (RingProducer, RingConsumer) pair. These classes have the respective enqueue/dequeue operations exposed.
• Adds a type class constraint for the size pointers in RingBuffer. As a compelling example, XDP requires ring buffers to use 32-bit sizes.
• Creates the trait IntSize which allows RingBuffer to juggle various size types (no wider than usize).
• Changes the memory ordering to release-acquire semantics, which are fully sufficient for a SPSC model.
• Re-exposes the ring as a public export from collections mod.

[ppenna]

Looks great. Thanks for working on this.

I left some minor suggestions.