Parsing and infrastructure support for SMT-LIBv2.5 input and output languages.

* support for new commands meta-info, declare-const, echo, get-model,
  reset, and reset-assertions
* support for set-option :global-declarations
* support for set-option :produce-assertions
* support for set-option :reproducible-resource-limit
* support for get-info :assertion-stack-levels
* support for set-info :smt-lib-version 2.5
* ascribe types for abstract values (the new 2.5 standard clarifies that
  this is required)
* SMT-LIB v2.5 string literals (we still support 2.0 string literals when
  in 2.0 mode)

What's still to do:
* check-sat-assumptions/get-unsat-assumptions (still being hotly debated).
  Also set-option :produce-unsat-assumptions.
* define-fun-rec
* All options should be restored to defaults with (reset) command.