Add predicate elimination as a new preprocessing step

[quickbeam123]

Implements the technique of Khasidashvili and Korovin (SAT 2016): a predicate P occurring at most once in every clause is eliminated by replacing S_P and S_~P with all pairwise resolvents on P. On problems without equality and theories, resolvents are computed with an mgu and non-unifiable pairs dropped; otherwise the P-literals are (virtually) flattened, introducing argument disequalities which are then simplified away by exhaustive equality substitution (this can introduce equality into a problem previously without it). FMB forces the equational mode, since its model reconstruction cannot rely on the Herbrand-interpretation argument justifying the mgu variant.

Elimination steps are gated SAT-VE-style by growth limits on the estimated clause count |S_P|*|S_~P| - |S_P| - |S_~P|: a global cap relative to the original total (-peltl, default 2.0). Syntactic tautologies (complementary pair, s = s) and duplicate literals are removed from generated resolvents, and the actual surviving count feeds back into the budget.

The next predicate to eliminate is by default the one with the smallest estimated growth (pure predicates thus go first, their clauses being simply deleted)

With -pels, the clause set is kept forward-inter-subsumed throughout, using a standalone LiteralSubstitutionTree index (unit literal, or the least matchable one) plus SATSubsumptionAndResolution; backward subsumption is left as future work.

Every elimination records a model-repairing definition
P(xs) <=> /_{D / P(ts) in S_P} exists ys. (xs = ts /\ ~D)
via Problem::addEliminatedPredicate (addTrivialPredicate for pure ones), so both the textual model updates output and FMB model restoration stay correct (verified via --mode model_check).

Skipped for higher-order/polymorphic inputs (predicates could hide inside terms, breaking the occurrence counting) and for color-annotated problems.

[quickbeam123]

The original commit message is preserved in the commit but describes more than what I eventually decided to keep. (I used interactive rebasing and "fixup" to hide my update commits, not sure if for the better.)

A TPTP FOL discount comparison is now:

Sort by SAT
1025 ['problemsSTD_pel10964_dis10_i10K.pkl']
1073 ['problemsSTD_pel10964_dis10_pel_i10K.pkl']

Sort by UNS
8406 ['problemsSTD_pel10964_dis10_i10K.pkl']
8591 ['problemsSTD_pel10964_dis10_pel_i10K.pkl']

Total 1108 / 8826

Num only-by (UNS):
   problemsSTD_pel10964_dis10_i10K.pkl 235
   problemsSTD_pel10964_dis10_pel_i10K.pkl 420

Greedy cover (UNS):
# problemsSTD_pel10964_dis10_pel_i10K.pkl contributes 8591 total 8591
# problemsSTD_pel10964_dis10_i10K.pkl contributes 235 total 8406
Total 8826

Overall, I already reviewed this and as a separate module in preprocessing, this shouldn't cause any problems.