Explicitly cast to uint32_t and suppress warnings by VC

```
../src/variable.c(1440): warning C4244: 'initializing': conversion from 'double' to 'uint32_t', possible loss of data
242
../src/variable.c(1470): warning C4244: 'initializing': conversion from 'double' to 'uint32_t', possible loss of data
243
```

TODO: check for `newsize` overflow

variable.c

@@ -1434,10 +1434,11 @@ rb_ensure_generic_iv_list_size(VALUE obj, uint32_t newsize)
     return ivtbl;
 }
 
+// @note May raise when there are too many instance variables.
 void
 rb_init_iv_list(VALUE obj)
 {
-    uint32_t newsize = rb_shape_get_shape(obj)->iv_count * 2.0;
+    uint32_t newsize = (uint32_t)(rb_shape_get_shape(obj)->iv_count * 2.0);
     uint32_t len = ROBJECT_NUMIV(obj);
     rb_ensure_iv_list_size(obj, len, newsize < len ? len : newsize);
 }
@@ -1467,7 +1468,7 @@ rb_obj_ensure_iv_index_mapping(VALUE obj, ID id)
 
     uint32_t len = ROBJECT_NUMIV(obj);
     if (len <= index) {
-        uint32_t newsize = (shape->iv_count + 1) * 1.25;
+        uint32_t newsize = (uint32_t)((shape->iv_count + 1) * 1.25);
         rb_ensure_iv_list_size(obj, len, newsize);
     }
     RUBY_ASSERT(index <= ROBJECT_NUMIV(obj));