Log Message:
Update to happy-1.20.0

* Fix #121: the -i flag produces an .info file even if
  the %expect pragma is violated
* Fix #131: qualify uses of Prelude functions in generated code
* Fix #161: drop fewer parse items when generating an .info file
* Introduce the %shift directive to resolve shift/reduce
  conflicts explicitly, useful in conjunction with %expect 0
* Remove the deprecated build configuration flag 'small_base'