Reduce UART Instance trait to a single function

[bugadani]

A bit of an experimental change, that allows (though doesn't fully implements) erasing the UART types completely in internal code.

At this point we can make UART's Instance completely public, allowing third parties to write their own drivers without relying on hidden code. Because there is no public, but hidden code :)

The driver itself is a bit of a mess, still, and I guess I'll need to write at least a changelog entry.

Build on #2406 though I guess we can simply close that PR as this one ended up being a bigger rewrite. I'll leave it open still since reviewing separately might be easier.