Improvements and fixes for symmetry detection and breaking (#2459)

Improvements and fixes for symmetry detection and breaking (#2459)

This fixes a few open issues with symmetry detection algorithm. It also extends the algorithm to do:
- Alpha equivalence to recognize symmetries between quantified formulas,
- A technique to recognize a subset of variables in two terms are symmetric, e.g. from x in A ^ x in B, we find A and B are interchangeable by treating x as a fixed symbol,
- Symmetry breaking for maximal subterms instead of variables.