Use polymorphic variants for phantom types

[talex5]

Instead of declaring a load of unique type names at the start of the file, use polymorphic variants. These don't need to be declared before use.

e.g.

type struct_Foo_11668594054516559475
type reader_t_Foo_11668594054516559475 = struct_Foo_11668594054516559475 reader_t
type builder_t_Foo_11668594054516559475 = struct_Foo_11668594054516559475 builder_t

module Foo : sig
  type t = reader_t_Foo_11668594054516559475
  type builder_t = builder_t_Foo_11668594054516559475
  val of_builder : builder_t -> t

becomes

module Foo : sig
  type struct_t = [`Foo_a1ef2c783fb1ce73]
  type t = struct_t reader_t
  val of_builder : struct_t builder_t -> t

This simplifies imports (all uses of Obj.magic have been removed from the generated code).

It also makes it possible to refer to a type before applying the generated code's functor. This might be useful for the RPC system, as it allows it to define a capability as something that takes a request struct, before applying the RPC schema's functor (that defines what a request struct is).

I also switched to formatting the node ID as hex, to make it shorter and to match the format used in the schema files.

Note that there is a slight API change here, because the $Struct.reader_t and $Struct.builder_t types have gone (to avoid confusion with the generic reader_t and builder_t types). You have to use struct_t reader_t and struct_t builder_t instead.

To make writing generic code simpler, there are two new helpers in StructStorage:

val reader_of_builder : 'a builder_t -> 'a reader_t
val message_of_builder : _ builder_t -> rw Message.t

This required some changes to the benchmark code, but the new code is shorter (6 files changed, 29 insertions(+), 57 deletions(-)).