This necessitates adding some extra definitions to the circuit header.
When standalone variables are used, we do not re-encode the entire
variable (which would take a miniumum of ~20 bytes per variable), but
instead just emit an index into the order that the variables were
defined at the top of the circuit.

At present, each control-flow scope will store the `Var` nodes anew, so
it's not a perfect only-store-once system.  This is consistent with how
registers are handled, though, and the QPY format isn't particularly
memory optimised as things stand.