Test nested circuit regions & command iterator.

[aborgna-q]

Make sure that nested HUGRs are supported by the Circuit interface.

Update the command iterator once everything is checked.
https://github.com/CQCL-DEV/tket2/pull/37#discussion_r1269154445

[aborgna-q]

This is quite relevant after #370. The commands iterator does not currently filter nodes from other regions, so in this example it tries to output the FuncDefn node:

graph LR
    subgraph 0 ["(0) Module"]
        direction LR
        subgraph 1 ["(1) FuncDefn"]
            direction LR
            2["(2) Input"]
            2--"0:0<br>qubit"-->4
            2--"1:1<br>qubit"-->5
            3["(3) Output"]
            4["(4) quantum.tket2.H"]
            4--"0:0<br>qubit"-->5
            5["(5) quantum.tket2.CX"]
            5--"0:0<br>qubit"-->3
            5--"1:0<br>qubit"-->6
            6["(6) quantum.tket2.T"]
            6--"0:1<br>qubit"-->3
        end
    end

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