Depend on ppxlib instead of (now deprecated) ppx_core, ppx_driver, ppx_metaquot, ppx_traverse and ppx_type_conv.
OCaml makes no distinctions between "foo" and {whatever|foo|whatever}. The delimiter choice is simply left to the user.
Do the same in our ppx rewriters: i.e. wherever we accept "foo", also accept {whatever|foo|whatever}.
Fix missing location in errors for broken custom printf example like:
printf !"%{sexp: int" 3;;Ppx_core evolution.