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/etc.c

@@ -23,7 +23,7 @@ mrb_data_object_alloc(mrb_state *mrb, struct RClass *klass, void *ptr, const str
 {
   struct RData *data;
 
-  data = mrb_obj_alloc(mrb, MRB_TT_DATA, klass);
+  data = (struct RData *) mrb_obj_alloc(mrb, MRB_TT_DATA, klass);
   data->data = ptr;
   data->type = (struct mrb_data_type *) type;