Generalize eager length bound conflicts for regular expression memberships (#7633)

Generalize eager length bound conflicts for regular expression memberships (#7633)

This generalizes eager length bound conflicts to take into account regular expression memberships.

For example:

If `(str.in_re x (re.++ (re.* re.allchar) (str.to_re "abc") (re.* re.allchar))` is asserted, then we know `(str.len x) >= 3`.

If e.g. equivalence class of `x` is merged with `(str.substr y 0 2)`, we get the conflict

`(and (str.in_re x (re.++ (re.* re.allchar) (str.to_re "abc") (re.* re.allchar)) (= x (str.substr y 0 2))`

since `(str.len (str.substr y 0 2)) <= 2`.

This also does some minor refactoring to eager prefix conflicts to make it more analogous to our implementation of length conflicts.