Specificity ordering for global context

[andrew4328]

An idea from Claude. Instead of checking subsumption on each function call site, it may be possible to sort the global context and just match the first unification. This is algorithmically complicated, because what are you supposed to do about disjoint unifications? You would need to be able to reliably identify those before the concrete argument types are visible.

Related: #1955 Prevent half-specialization

[andrew4328]

This can be implemented as a subsumption tree and linear scan.

Algorithm:

• linear scan for matches
• check subsumption tree to find if matches are disjoint

This moves the O(n^2) match set reduction algorithm from per function to per global.