[Rust] Simplify done unwrapping to prevent dual mutable aliasing

[ZackPierce]

Previously the done-state of generated rust encoders and decoders handed back a reference to the wrapped data buffer and a the final position read/written to. Now only the final position is provided.

This ensures that it is impossible for the mutable references data struct overlays optionally created during encoding to coexist with a same-lifetime'd mutable byte slice produced at the end. Since the final position is still returned, and the original input buffers were all slice-references anyhow, users should be able to trivally re-slice the source data equivalently themselves without loss of functionality.

A minor performance gain is also accomplished because returning a usize is marginally cheaper than returning a newly-allocated tuple containing a usize and a slice-reference.

@mjpt777