Add stats for string reductions, lemmas and conflicts (#4149)

Add stats for string reductions, lemmas and conflicts (#4149)

This PR adds comprehensive stats for reductions, lemmas and conflicts in TheoryStrings. Remaining stats will track all inferences (which are finer grained steps that may lead to lemmas/conflicts).

Additionally this PR refactors calls to OutputChannel::lemma in TheoryStrings / InferenceManager. There are now only 2 calls to lemma(...) during registerTerm(...), one for "atomic" string terms (corresponding to length splits typically) and one for non-atomic terms.