Release 1.8.0

Version 1.8.0

• Explicit Solver::clause functions to simplify clause addition.

• More fine-grained handling of printing proof size information by
  adding bool print = false flags to the flush_proof_trace and
  the close_proof_trace API calls. The former prints the number
  of addition and deletion steps, while the latter prints the size
  of the proof size (and the actual number of bytes if compressed).
  The main effect is that by default printing of proof size disabled
  for API usage but enabled for the stand-alone solver.