(PDOC-3) Rename FutureParser and ParserFunctions

Prior to this commit, the handlers and code objects which dealt with
puppet functions were referred to as `FutureParserFunctions` and
`ParseFunctions`. This naming convention will not make very much
sense going forward when the future parser becomes the primary
parser. Therefor, rename `FutureParserFunctions` to `Puppet4xFunctions`
and `ParserFunctions` to `Puppet3xFunctions`.