Qasm refactor

[Mayank447]

Pending qasm-refactor from #290. There are couple lots of pre-existing error in the qasm_processor.py file. Couple of variables are undefined e.g. curr_gate, reg_num and some are unused e.g. prev_token, gate_added (not sure whether some logic is missing or they are simply not needed).

@BoxiLi I haven't looked at the conversion logic properly as my first aim was to refactor the 1200 line single qasm.py file. I believe instead of fixing these errors as per OpenQASM 2.0 circuit conversion, we can directly upgrade to OpenQASM 3.0 (in accordance with #292) in this or another PR. Since OpenQASM 3.0 supports a lot of new features like loops, extern etc. the conversion logic would change anyways.

[BoxiLi]

Yes I agree there are some qutie complicated logic in qasm.py and many potential bugs. Basically we implemented our own parser for a qasm file. I think there should be a smarter way of doing it.

I am surley happy to upgrade to qasm 3.0, also not against completely rewrite it if you find it a better approach. Would be good to keep the support of 2.0 if possible, because some platforms still only supports that. But this is open for discussion.

[Mayank447]

Cool, I will work on resolving the bugs in openqasm 2. Unfortunately there is no parser for qasm 2 which converts .inc to an AST, so we will need to maintain a custom parser.

For qasm3 openqasm provides a python package for parsing, so that should make it easier to implement.