toplevel: find shorter path using the environment more directly

[Octachron]

ping @gasche

This sub-PR implements my proposition for using the data available in Env to find shorter path for constructor, rather than doing multiple requests to the environment on successively longer guesses.

[gasche]

@Octachron your PR does not seem to behave as I would expect in the following case (I will add this to my own PR as a regression test):

(* Introduce Q with a Sub submodule *)
module Q = struct
  module Sub = struct type t = A end
end

(* open Q: Q.Sub.A can now be printed as Sub.A *)
open Q

(* shadow the Sub module:
   Sub.A is not a valid printing choice for Q.Sub.A anymore *)
module Sub = struct end
[%%expect {|
module Q : sig module Sub : sig type t = A end end
module Sub : sig end
|}]

(* Test the printing of Q.Sub.A *)
let it = Q.Sub.A
[%%expect {|
val it : Q.Sub.t = Q.Sub.A
|}]

With your PR, at the end I get Sub.A, while I expect Q.Sub.A.

[gasche]

(Otherwise I find it hard to comment on the implementation because I don't really understand what is going on in TyCompTbl. I was surprised by the fact that it's a lot of code, while I expected it to be simpler than my own version. On the other hand, I suppose that it is more general and could be reused in other situations.)