Add equality comparisons for manual variables to DAGCircuit.__eq__

[jakelishman]

Ensure that DAGCircuit.__eq__ (and by extension QuantumCircuit.__eq__) works in the presence of manual variables. It is not immediately clear whether any sort of variable equivalence should be checked by default, but for the sake of conservatism in the initial implementation, we can say that only Var instances that directly compare equal will be considered equal (despite the expr.structurally_equivalent checker giving us the option to expand this). With expr.Var expected to contain a UUID field to facilitate comparisons between differently serialised variables (#10923), the main reason to allow a more semantic-equality checker is far less prominent.