Update Hamiltonian Circuits Example

[cyderize]

This changes Hamiltonian Circuits example to remove use of global variables.

The result list is just returned by the function rather than as a third parameter (but if doing that is clearer then I'll change it to be a parameter instead). I'm not sure if this is more or less confusing than before, though.

[stevenbird]

Thanks @cyderize – I prefer this style.