Hol_datatype ignores theory indications

[xrchz]

The following session (in theory Scratch) should not fail. It looks like the wrong datatype was defined, making scratch$num recursive when it was explicitly annotated not to be.

> val _ = Datatype`num = Foo (num$num) | Bar`;
<<HOL message: Defined type: "num">>
> ``Foo 0``;
at line 7, character 6: No possible type for overloaded constant _ inject_number
Wanted it to have type:  :num -> num
Exception-
   HOL_ERR
  {message =
  "at line 7, character 6:\nNo possible type for overloaded constant _ inject_number\n",
  origin_function = "typecheck", origin_structure = "Preterm"} raised
> dest_thy_type(fst(dom_rng(type_of``Foo``)));
val it = {Args = [], Thy = "scratch", Tyop = "num"}:
   {Args: hol_type list, Thy: string, Tyop: string}