Ensure register decls and usage match

[qartik]

A possible "gotcha" here is that if a pytket circuit contains bits that do not form part of a full register -- for example, if it contains a bit c[1] but not c[0] -- then these bits are ignored by circuit.c_registers (similarly for qubits). I am not sure how phirgen handles this case currently (maybe an error, maybe add missing bits?) -- but if such a circuit were passed to this function it would presumably generate invalid PHIR.

[...]

Sure, how about this:

from pytket.circuit import Circuit, Qubit, Bit
from pytket.phir.api import pytket_to_phir

circ = Circuit()
circ.add_qubit(Qubit(1))
circ.add_bit(Bit(1))
circ.H(1)
circ.Measure(1, 1)
phir = pytket_to_phir(circ)
print(phir)

Currently this produces the following:

{
  "format": "PHIR/JSON",
  "version": "0.1.0",
  "metadata": { "source": "pytket-phir v0.8.1" },
  "ops": [
    {
      "data": "qvar_define",
      "data_type": "qubits",
      "variable": "q",
      "size": 1
    },
    { "data": "cvar_define", "data_type": "i64", "variable": "c", "size": 1 },
    { "//": "H q[1];" },
    { "qop": "H", "angles": null, "args": [["q", 1]] },
    { "//": "Measure q[1] --> c[1];" },
    { "qop": "Measure", "returns": [["c", 1]], "args": [["q", 1]] }
  ]
}

This looks incorrect, since it seems to be indexing q[1] in a size-1 register. I am not sure what pecos will do with this, probably error?

With the change in this PR, I suspect it will be differently wrong, since the qubit and bit registers would not be declared at all?

I think it would be OK to reject circuits with incomplete registers.

Originally posted by @cqc-alec in #238 (comment)