DO NOT MERGE: polishing experiments

[MabezDev]

I suggest bulding and viewing the docs locally, then reviewing this commit by commit. The intention is not to merge this, but choose what we want from it and I'll PR them seperately.

• Commit 1 should be fairly uncontroversial I think. I don't think the split API is fully ready yet, and we may want to experiment more here.
• Commit 2 replaces write|read_bytes with just write|read when the input parameter is a byte slice. Imo this is fine, and there is precidence in the std (e.g https://doc.rust-lang.org/std/fs/fn.write.html only accepts bytes)(and other crates in the embedded ecosystem for this)
• Commit three shuffles some things around to firstly show stable items for uart, then unstable, to complete this I would apply this to all partially stable drivers.

Comments appreciated.