Upgrade Menhir (was: bump p4pp version)

[hackedy]

This should fix the menhir-related build issues we've been having.

[hackedy]

Summary of changes

• Require p4pp v0.1.8, which works on new versions of menhir
• Require menhir 2.0 in dune instead of menhir 1.0
• Require dune 2.8 in dune-project
• Rename Info -> P4info
• Remove module aliasing tricks to handle Info conflict with an Info module in Core
• Rename Num constructor to Index to avoid name conflict in lib/ast.ml (the stf-test ast)
• Add annotations in parser.mly where menhir type inference was failing
• Add fully qualified names in parser.mly where menhir type inference was failing
• remove unused open Checker_env in lib/checker.ml

[QinshiWang]

What do we need to install the new version?

[hackedy]

Running opam update and then opam install . --deps-only should do it