Update extension inference to use subset constraints

[croyzor]

Reference: extension reredux

The current unification system for extension inference uses equality constraints between metavariables. We allow target ports to have a greater set of extension requirements than the source ports that are wired to them, and we represent this by adding Lift nodes to those wires.

We've decided that these Lift nodes aren't worth the effort, so we should switch to representing our constraints directly as subsets rather than equality. I.e. we create a constraint saying that the requirements of a node are less than or equal to another node.

Replace the current version of the extension inference unification algorithm with the redux version which instead has constraints standing for subsets. We should then be able to delete lift nodes entirely.