Automatic generation of runtime types from type definitions.

This syntax extension defines the type-conv generator [@@deriving typerep], which
creates a (runtime) value (called typerep_of_$typename) representing the type definition
(see typerep for more information). It is intended to be the main creator of values of
type Typerep.t.

This generator supports mostly core types, not all fancy types like union of polymorphic
variants.

A note about signatures

In signatures, ppx_typerep_conv tries to generate an include of a named
interface, instead of a list of value bindings.
That is:

type 'a t [@@deriving typerep]

will generate:

include Typerepable.S1 with type 'a t := 'a t

instead of:

val typerep_of_t : 'a Typerep_lib.Std.Typerep.t ‑> 'a t Typerep_lib.Std.Typerep.t
val typename_of_t : 'a Typerep_lib.Std.Typename.t ‑> 'a t Typerep_lib.Std.Typename.t

There are however a number of limitations:

• the type has to be named t

• the type can only have up to 3 parameters

• there shouldn't be any constraint on the type parameters

If these aren't met, then ppx_typerep_conv will simply generate a list of value
bindings.

Weird looking type errors

In some cases, a type can meet all the conditions listed above, in which case the
rewriting will apply, but lead to a type error. This happens when the type [t]
is an alias to a type which does have constraints on the parameters, for
instance:

type 'a s constraint 'a = [> `read ]
val typerep_of_s : ...
val typename_of_s : ...
type 'a t = 'a s [@@deriving_inline typerep]
include Typerep_lib.Typerepable.S1 with type 'a t := 'a t
[@@@end]

will give an error looking like:

Error: In this `with' constraint, the new definition of t
       does not match its original definition in the constrained signature:
       Type declarations do not match:
         type 'a t = 'a t constraint 'a = [> `read ]
       is not included in
         type 'a t
       File "typerepable.mli", line 11, characters 2-11: Expected declaration
       Their constraints differ.

To workaround that error, simply copy the constraint on the type which has the
[@@deriving] annotation. This will force generating a list of value bindings.