more typecheck listeners

[digama0]

Continuation of #1346. Adds typecheck listeners for

• pretype elaboration, i.e. “:type” and Type foo = ...
• Definition elaboration
• Datatype elaboration
  • This also has a parse listener because simple datatypes skip the typechecker

[mn200]

I think the codebase should make it easy for multiple listeners to coincide in the one "slot". Could perhaps have

signature listener = 
sig
 
   type 'a t
   val add_listener : 'a t -> (string * ('a -> unit)) -> unit
   val remove_listener : 'a t -> string -> unit
   val current_listeners : 'a t -> (string * ('a -> unit)) list
end

which would be implemented with a ref to a list of listener functions (of type 'a -> unit)
You might then use with

signature foo = sig
  ...
  val event_listener : eventtype listener.t

end

We avoid exposing the implementation as refs, and stop people from foolishly dumping over existing entries.

[mn200]

Happy to merge as is for the moment anyway.