Add ability to report state in simulation backends

[jake-arkinstall]

Simulators know their underlying state. Users should be able to query this state within guppy code for debugging purposes.

Because of the following:

• a point in guppy code one might not be aware of all of the currently allocated qubits, e.g. in recursion (the number of allocated qubits might depend on the current recursion level)
• the user will also want to specify some ordering of qubits
• it isn't generally valid to trace out unrequested qubits

Then it makes sense to make the call look like:

q1, q2 = qubit(), qubit()
do_the_thing(q1, q2)
report_state("some_tag", [q1, q2])

This should output the state with the Most* Significant Bit representing q1, the second Most Significant Bit representing q2. Any other qubits will occupy further bits.

*or least, I have no preference.

[cqc-alec]

If we are representing the state as a dictionary rather than a vector then the MSB/LSB question becomes moot. We just preserve the order of the qubits in each of the keys of the dictionary.