More C++ compilability work: mrb_obj_alloc void* conversions

One of the biggest set of changes needed to make C++ compile, is that you
can't autoconvert "void*" to a different pointer type without a cast (you
can of course, convert pointers *to* "void*"!)

For the first part, convert the users of "mrb_obj_alloc".  Since it has
to return something, make it RBasic* (that's what mrb_obj_alloc() is
operating on anyway).  This way, even in C you'll get a warning if you
don't cast it.

For places where there are a lot of similar calls to mrb_obj_alloc(),
this can be easily hidden through a macro.  I did this in string.c:
    #define mrb_obj_alloc_string(mrb) ((struct RString *) mrb_obj_alloc((mrb), MRB_TT_STRING, (mrb)->string_class))

I also updated the mrb_object() macro to also return a RBasic* -- my
previous commit changed that from "void*" -> "RObject*", but I figure
it should be consistent with mrb_obj_alloc()

src/error.c

@@ -183,7 +183,7 @@ exc_equal(mrb_state *mrb, mrb_value exc)
 void
 mrb_exc_raise(mrb_state *mrb, mrb_value exc)
 {
-    mrb->exc = mrb_object(exc);
+    mrb->exc = (struct RObject *) mrb_object(exc);
     longjmp(*(jmp_buf*)mrb->jmp, 1);
 }