[ScfToCf] Signedness-aware scf::ForOp lowering

[lucas-rami]

This introduces a new conversion pass to Dynamatic++ (--lower-scf-to-cf) that is almost identical to the upstream --convert-scf-to-cf pass, which, as its name indicates, converts scf operations to unstructured control flow (cf). The only difference between the two is that the scf::ForOp rewrite pattern used in our pass (which overwrites the default one) inserts an unsigned comparison operation for checking whether the loop iterator is within the for loop's bounds instead of a signed one if it can prove that both the iterator and upper bound are guaranteed to be positive. The estimation relies on the recently introduced numerical analysis. This has a performance impact down-the-line during bitwidth analysis.

Fixes #28.