Optimise references storing integers.
Optimise references storing integers.

By making the value stored inside the reference mutable, we avoid excessive
allocations.

xhava/hippyvm,xhava/hippyvm,hippyvm/hippyvm,hippyvm/hippyvm,hippyvm/hippyvm,xhava/hippyvm,hippyvm/hippyvm,hippyvm/hippyvm,hippyvm/hippyvm,xhava/hippyvm,xhava/hippyvm,xhava/hippyvm,xhava/hippyvm,hippyvm/hippyvm