Revert "[RDF] Allow RegisterRef to contain register unit" and follow-ups

This reverts commit 859b05b.

Also reverts these follow-ups:

Revert "[RDF] Remove `constexpr` from `hash"
This reverts commit 621507c.

Revert "[RDF] Do not use trailing return type after all, NFC"
This reverts commit 46e19e3.

Revert "[RDF] Stop looking when reached code node in getNextRef with NextOnly"
This reverts commit a049ce9.

Revert "[RDF] Use trailing return type syntax, NFC"
This reverts commit d3b34b7.

Revert "[RDF] Define short type names: NodeAddr<XyzNode*> -> Xyz, NFC"
This reverts commit f8ed60b.