Strings: Strengthen multiset reasoning (#2817)

Strings: Strengthen multiset reasoning (#2817)

This commit introduces three helper methods for performing multiset
reasoning: an entailment check whether a term is always a strict subset
of another term in the multiset domain (`checkEntailMultisetSubset()`),
a check whether a string term is always homogeneous
(`checkEntailHomogeneousString()`), and an overapproximation for the
multiset domain (`getMultisetApproximation()`). It also adds unit tests
related to multiset reasoning.