Using a single condition enumerator in sygus-unif (#2440)

Using a single condition enumerator in sygus-unif (#2440)

This commit allows the use of unification techniques in which the search space for conditions in explored independently of the search space for return values (i.e. there is a single condition enumerator for which we always ask new values, similar to the PBE case).

In comparison with asking the ground solver to come up with a minimal number of condition values to resolve all my separation conflicts:

- _main advantage_: can quickly enumerate relevant condition values for solving the problem
- _main disadvantage_: there are many "spurious" values (as in not useful for actual solutions) that get in the way of "good" values when we build solutions from the lazy trie.

A follow-up PR will introduce an information-gain heuristic for building solutions, which ultimately greatly outperforms the other flavor of sygus-unif.

There is also small improvements for trace messages.