Add manual variable support to expr.Var

[jakelishman]

Add support to expr.Var for representing classical variables with some fixed storage location as described in Qiskit/RFCs#50. For the initial pass, we can use some UUID as the backing "data store" link value to use in direct comparisons, which has the advantage of letting QPY round-trip serialisation reproduce the values exactly, whereas requiring singleton object identities for equality has the problem that two circuits serialised separately will end up with spuriously difference variables.