support output assignment for function calls

[yitchen-tim]

closes: #69

Description of change:
Support optional output assignment for oqpy.function_call

[PhilReinhold]

So there are a few existing options for this. I think the "most correct" way is to use an extern, i.e.

my_function = oqpy.declare_extern("my_function", [], oqpy.bit)
b = oqpy.BitVar(name="b")
prog.set(b, my_function())

If you really don't want the extern statement to be added you can say "include_externs=False" when generating the program.

You could also do this without creating an extern via prog.set(b, oqpy.OQFunctionCall("my_function", [], oqpy.bit)).

That being said, maybe it is just easier to add this extra code to allow another way of doing it.

[yitchen-tim]

thanks for the suggestion! A follow up question: what is the purpose for program.function_call? I assume it's for calling build in functions without first defining them like externs. In the future, if a oqpy built-in function call requires return values, doesn't oqpy still need to implement this PR?

[PhilReinhold]

thanks for the suggestion! A follow up question: what is the purpose for program.function_call?

It is an escape hatch to allow manual workaround, it shouldn't be required in normal operation, but yes I think as long as we have it, we should allow it to assign to variables, so I think this change is good. On second look, I don't have any requested changes, so I think we can just merge this, sorry for the perhaps unnecessary delay.

[yitchen-tim]

Understood. I think the corner cases that require using function call but not extern would be someone want to make a function call without producing an extern statement in OpenQASM script, but they want to use include_externs=True for other extern statements.