Pick some low-hanging circuit optimization fruit

• Use Seq instead of [] in And and Or.
• In (&&), (||), xor and choose, add cases for constant false/true
  parameters.
• Dedicate the literals -1/1 for the constants false/true. Drop Lit.
  Always assert 1.

The regexp-grid example went from 71737 literals and 427725 clauses to
8618 literals and 172100 clauses. Both formula generation and solving
got noticeably faster.