Seq rewriter integration

[cdstanford]

(Redoing pull request)

Integrate the new derivative normalization functionality into the sequence rewriter.

The primary changes are:

• All conditions formed when creating a derivative now go through the mk_der_cond function, which enforces a desired normal form. Currently, it enforces that conditions are rewritten to the form x <= c where c is a character code.

• In the BDD form, conditions are order by character code whenever possible; otherwise expression ID only as a fallback. So, we redefine get_id in the context of mk_der_op_rec to be c for conditions of the form x <= c.

• In the BDD form, conditions are then simplified locally: unreachable branches are removed. (This uses the auxiliary function pred_implies)

The result is that for regexes which only involve ranges and raw characters/strings as predicates, the BDD form is linear and all leaves of the BDD are reachable. Finally:

• Additional tracing under the seq_verbose tag