(PUP-3274)(PUP-3638) Make future parser standard and remove 3x parser/lexer/ast

This is a mega removal and cleanup of old 3x code.

* Removal of lexer, parser, and the majority of the AST classes (except those still used by the compiler
* Removal of related tests; merging future and old tests that should remain
* Removal of parser setting
* Removal of binder setting
* Cleanup of remaining AST classes
* Adjusting documentation since "future" is now here
* Fixing benchmarks that were specific for future parser
* (PUP-3638) Removal of RDoc1
* (PUP-3638) Removal of RDoc2 generation of documentation for all .pp
* Removal of doc support in the remaining AST classes
In the spec/unit/functions4_spec.rb these two lines re puppet/plugins are deleted.  Should they be removed here as well?