Support for visiting OpenPulse statements

[kshyatt-aws]

The "parsing" step to generate QasmExpressions for OpenPulse is working with the examples in test/runtests.jl, but the "visiting" step to generate output that can be digested by other packages isn't done, partly because I'm unsure what the final output should be (e.g. what's the equivalent of CircuitInstruction for waveforms? Frames?). Would be great to have someone with a little more familiarity with OpenPulse work on that.