Statements like `c = measure q` are analyzed correctly.
But `c[0] = measure q` is not analyzed correctly. The code
that builds the assignment statement assumes the LHS is an identifier.
The ASG should be able to handle valid lvalues. But we cannot yet
construct statements with them.

This is done twice and is several lines, so it is a function now.

This is unrelated to measurement.

`GateOperand` is a qubit argument to gate call. And arguments to
`measure` have the same spec. Identifier, hardware identifier,
or indexed identifier. So we reuse the code from gate call.

The code in oq3_parser is a bit mysterious. I added some comments to
make it easier to locate next time, I hope.