Introduce best content heuristic for strings (#4382)

Introduce best content heuristic for strings (#4382)

* Introduce best content heuristic for strings

This commit introduces a "best content heuristic" to perform
context-dependent simplifications. The high-level idea is that for each
equivalence class for strings, we compute a representation that is a
string concatentation of constants and other string terms. For this
representation, we try to get as many letters in the string constants as
we can (i.e. the best approximation of the content). This "best content"
representation is then used by `EXTF_EVAL` to perform simplifications.

Co-authored-by: Andrew Reynolds <andrew.j.reynolds@gmail.com>