fix .constructor when disowning

[penalosa]

Apply the fix in #10 to disown functions.

Without this change, we were seeing generated TS files like:

  _disownConstructor(): $.Orphan<Constructor> {
    return $.utils.disown(this.constructor);
  }

which failed with:

Argument of type 'Function' is not assignable to parameter of type 'Pointer<_Pointer>'.
  Type 'Function' is missing the following properties from type 'Pointer<_Pointer>': _capnp, byteOffset, segment, [Symbol.toStringTag]
Error: CAPNP-ES009 Failed to transpile emitted schema source code; see above for error messages.