feat: refactor to allow for split mode of operation

[mmastrac]

Works towards solving #40, allowing read and write halves to work independently. Maintains the old API for compatibility.

This is a pre-factoring PR that splits out a ReadHalf and WriteHalf, and ensures that you can read and write independently. A later PR will build on this to create the split API.

The main parts of this PR are:

• A new Fragments struct that collects Frames.
• The split ReadHalf and WriteHalf that do not contain a stream, allowing us to drive reads and writes externally
• Reads provide an 'obligated write' which the caller is required to send

I tried to avoid making any major logic changes to functions -- this is mainly moving things around for a later patch.

Also note that the logic around checking whether the WriteHalf is closed or not has been hoisted into the read callers. This is a little awkward given that there is a fragmented and non-fragmented read path that duplicate this logic, but we may be able to clean this up in the future.

[M4urici0GM]

That's awesome, actually, i need this feature as well. Hope it gets merged 🚀