Signature mismatch errors use OCaml wording #9066

Signature mismatch errors still use OCaml wording in several places:

• Variant constructors are called fields:

  module M: {
    type rec t<_> = Int(int): t<int> | Str(string): t<string>
  } = {
    type rec t<_> = Int(int): t<int> | Str(string): t<int>
  }

  The types for field Str are not equal.
  

  The same holds for "The arities for field A differ", "Fields number 2 have different names" and "The field C is only present in the first declaration" when comparing variants.

• Functor positions are written in OCaml syntax:

  module type S = {
    type t
  }
  module M: {
    module F: (X: S) => S with type t = X.t
  } = {
    module F = (X: S) => {
      type t = int
    }
  }

  At position module F(X) : <here>
  

• Names are quoted OCaml-style: The value `b' is required but not provided.

• Extension constructors are printed with two spaces: type t += Boom(string).