Ppx_ast selects a specific version of the OCaml Abstract Syntax Tree from the
migrate-parsetree project that is not necessarily the same one as the one
being used by the compiler.

It also snapshots the corresponding parser and pretty-printer from the OCaml
compiler, to create a full frontend independent of the version of OCaml.

This AST is used in all Ppx_core/Ppx_driver based ppx rewriters.