[P4Testgen] Hotfixes and improvements to the P4Constraints parsers.

[fruffy]

• Add a small builtin map to the refers_to parser. Builtin objects are prefixed by builtin::
• Improve the readability and error handling of the refers_to parser.
• Fix a crash in the entry_restriction parser when comparing to a negative constant.
• Add support for referenced_by and action_restriction.
• Add some type aliases.