fix another size-recalculation mistake in 'PersistentVector.splitRight' function. (When we are splitting in the middle of a BRANCH, we want to subtract the old child size from the remainder, and then add the new child size to the remainder, because this calculation will account for the splitting of preceding elements.)

fcore/persistent-vector.sml

@@ -450,13 +450,16 @@ struct
             else
               let
                 val newChildSize = getFinishIdx child
-                val difference = oldChildSize - newChildSize
                 val sizes = Vector.tabulate (VectorSlice.length sizesSlice + 1,
                   fn i =>
                     if i = 0 then
                       newChildSize
                     else
-                      VectorSlice.sub (sizesSlice, i - 1) - difference
+                      let
+                        val el = VectorSlice.sub (sizesSlice, i - 1)
+                      in
+                        el - oldChildSize + newChildSize
+                      end
                 )
 
                 val child = VectorSlice.full #[child]