Prior to this commit, several rspec tests were using `decribe "string"'
instead of `describe constant`. Update the tests to use the constant instead.
Additionally, rename the puppet function tests to reflect the renaming of the
handlers.
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`.
Hailee Kenney