Cleanup interface for PTS.Interactive
Update REPL API and docs

* Take PTS instance as 1st arg
* Fix out-of-date code in DEVELOPMENT.md (we should automate testing of this)
* Expose state produced by processStmt, since there's no interesting
  return value. This means de-inlining some wrappers I just inlined :-(,
  but that's a local thing and with a clear purpose :-).
* Document how to lens out the interesting stuff