Eliminate warnings in lexer (#18)
Eliminate warnings in lexer (#18)

The p4-hlir lexer currently prints warnings every time it is invoked.

> WARNING: No error rule is defined for exclusive state 'pragma'
> WARNING: Token 'PPHASH' defined, but not used
> WARNING: There is 1 unused token

This commit supresses those warnings by:

* Eliminating the PPHASH token, while retaining the `t_PHASH` rule so
  location information encoded in `#line` directives is correctly tracked.

* Adding a `t_pragma_error` rule.
p4lang/p4-hlir