WIP: QASM3 Compilation

[gribeill]

Rather than develop our own full-stack quantum circuit compiler and optimizer, we can use the relatively widely used QASM language to interface with other toolkits like QisKit and Cirq. The 3.0 release of QASM is a good match to QGL's capabilities for mixed classical/quantum control. It should be possible to compile many QASM3 programs into QGL sequences (up to what the sequencer supports).

A rough roadmap:

• [x] QASM3 lexer for tokenization
• [x] Parse tokens into CST
• [ ] Various transformations to go from CST -> AST
• [ ] Gate translation (from QASM U representation into concrete pulse library)
• [ ] Realtime function mapping (QASM bit to sequencer VRAM) and control flow
• [ ] Tests tests tests!

Some open questions:

• Should we optimize gates at this level? We can re-use some of the RandomCircuits.jl infrastructure for compressing single qubit gates and decomposing 2-qubit gates.
• How best to deal with subroutines and loops? Should we do some amount of inlining/unrolling?
• How to deal with classical instructions that aren't implemented (should we implement some of these?)