Get [@@deriving]-style generation of type-directed values without writing a ppx

Ppx_type_directed_value is a ppx that does [@@deriving]-style generation of type-directed values based on user-provided modules. The user-provided modules tell ppx_type_directed_value how to compose type-directed values (for example, combine type-directed values of the fields of a record to form a type-directed value for the record itself).

This allows a wide variety of PPXs such as ppx_sexp_conv, ppx_compare, ppx_enumerate, etc. to be implemented with ppx_type_directed_value, but with some runtime cost.

This PPX currently supports deriving type-directed values for records, ordinary & polymorphic variants and tuples. It also supports custom user-defined attributes on record and variant fields.