Depend on ppxlib instead of (now deprecated) ppx_core and ppx_driver.
Open_in_bodysupport from ppx_let, since it was only ever used
in confusing chains of
Let_syntaxmodules that introduced other
Let_syntaxmodules in the "body" (e.g. for defining Commands whose
bodies use Async). In this case it was decided that the better
practice is to be explicit with
open ___.Let_syntaxat the different
transition points, even though this is more verbose.