prepend error caused by incorrect sizes

[cbfiddle]

I'm not sure if anyone cares about this code, but there appears to be an error at line 2866 of rrbvector. When this code determines that the node is balanced, it should clear the sizes entry, but instead it leaves the old sizes array in place.

[nicolasstucki]

I'm not actively maintaining this codebase but this will be useful for anyone planning to reuse the code or concepts from this repo.