Generalize concat conflict for sequences in LFSC (#8625)

Generalize concat conflict for sequences in LFSC (#8625)

This ensures LFSC proofs are correct when using CONCAT_CONFLICT for sequences.

In detail, to justify why (seq.++ (seq.unit c1) t1) = (seq.++ (seq.unit c2) t2) is a conflict, where c1 and c2 are constants, we require an explicit step to evalute (seq.unit c1) = (seq.unit c2) to false, since c1 and c2 are theory-specific constants.

Note this is different from (str.++ (char n1) t1) = (str.++ (char n2) t2) where a syntactic check of n1 and n2 suffices.

This fixes proof checking for regress0/seq/seq-types.smt2.