Adding large-step encoding for CHCs in CProver (no [required] changes to the solver)

[yvizel]

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[tautschnig]

A bunch of comments that each apply in several more places. More generally, however, it would be great to have comments. At bare minimum there should be high-level design comments in some place that explain how the various pieces fit together. Also, we need tests that demonstrate the new feature.

[tautschnig]

I'd suggest that used_relations just return a std::vector<symbol_exprt> (which is cheap with copy elision).