Remove ppxlib dependency from entire monorepo
- prune: Replace ppxlib with compiler-libs.common, converting
Ast_traverse.iter visitors to Ast_iterator.default_iterator
- prune: Adapt to OCaml 5.3 Longident changes (located strings)
- Remove ppxlib from root dune-project, prune dune-project and opam
- Clean up stale ppxlib references in merlint comments and docs