[opengoal] make none a child of object (#3001)

Previously, `object` and `none` were both top-level types. This made
decompilation rather messy as they have no LCA and resulted in a lot of
variables coming out as type `none` which is very very wrong and
additionally there were plenty of casts to `object`. This changes it so
`none` becomes a child of `object` (it is still represented by
`NullType` which remains unusable in compilation).

This change makes `object` the sole top-level type, and the type that
can represent *any* GOAL object. I believe this matches the original
GOAL built-in type structure. A function that has a return type of
`object` can now return an integer or a `none` at the same time.
However, keep in mind that the return value of `(none)` is still
undefined, just as before. This also makes a cast to `object`
meaningless in 90% of the situations it showed up in (as every single
thing is already an `object`) and the decompiler will no longer emit
them. Casts to `none` are also reduced. Yay!

Additionally, state handlers also don't get the final `(none)` printed
out anymore. The return type of a state handler is completely
meaningless outside the event handler (which is return type `object`
anyway) so there are no limitations on what the last form needs to be. I
did this instead of making them return `object` to trick the decompiler
into not trying to output a variable to be used as a return value
(internally, in the decompiler they still have return type `none`, but
they have `object` elsewhere).

Fixes #1703 
Fixes #830 
Fixes #928

test/decompiler/test_FormExpressionBuild3.cpp

@@ -161,6 +161,6 @@ TEST_F(FormRegressionTestJak1, WeirdShortCircuit2) {
       "    jr ra\n"
       "    daddu sp, sp, r0";
   std::string type = "(function actor-link-info object)";
-  std::string expected = "(the-as object (and (-> arg0 prev) (-> arg0 prev extra process)))";
+  std::string expected = "(and (-> arg0 prev) (-> arg0 prev extra process))";
   test_with_stack_structures(func, type, expected, "[[16, \"event-message-block\"]]");
 }