Output language "cvc3" (as opposed to "cvc" or "cvc4") produces output for CVC3:

1. no decimals used for rational literals
2. queries/check-sats wrapped with PUSH/POP
Add LETP (parallel let) to CVC4 native language.

Change CVC4 printer to use LETP by default.  CVC3 printer has no LETP.
DAG output fixed to support quantifiers and lambdas (resolves bug #358).