Remove cruft that's no longer needed after pratt-jep-4

The pratt-jep-4 changed the internal AST of projections.  Several
of the AST nodes are now no longer needed.  I've also removed any
of the AST unit tests related to those nodes.  The compliance tests
have those covered anyways.

I also removed anything related to generated the lrtable's as we're
no longer using PLY.

And finally, we don't need to pull a dependency on PLY anymore.
Disallow '@' as a top level syntax

The grammar does not allow '@' as a bare token.  It's only allowed
in a function call.
Merge branch 'cleanup-pratt-jep-4' into develop

* cleanup-pratt-jep-4:
  Remove cruft that's no longer needed after pratt-jep-4
Merge branch 'no-allow-current-node' into develop

* no-allow-current-node:
  Disallow '@' as a top level syntax