Add support for most OpenQASM 2 and 3 standard gates.

[dlyongemallo]

Adds support for the most common OpenQASM 2 and 3 standard library gates, and a unit test to ensure that the implementations in pyzx and qiskit are the same up to a global phase.

This PR fixes #153 and #158, adds tests to verify #102 and #103, and should now go most of the way towards completing #87 and #116.

[dlyongemallo]

@jvdwetering I'm slightly unsure about whether to_basic_gates() for crx and rxx are correct. Does the list which is returned have to consist only of gates for which to_basic_gates() return themselves, or is to_basic_gates() defined recursively?

[dlyongemallo]

There are a few outstanding gates in qelib1.inc and stdgates.inc which haven't been included. I wanted to make sure that these are correct first, before I implement those.

[jvdwetering]

to_basic_gates should itself return a list of basic gates, so it should not have to be called recursively.

[dlyongemallo]

I fixed the to_basic_gates for the newly added gates.

The only remaining gates which are in the OpenQASM standard libraries which aren't implemented yet are: rccx, rc3x, c3x, c3sqrtx, and c4x. I can add them later, or I can add them in this PR if you think they're necessary.

[jvdwetering]

I've never seen anyone use those gates, so I think it's fine if these are not implemented for now.

[jvdwetering]

Can this be merged?

[dlyongemallo]

AFAIK, yes. Unless you have any comments or requests about the code.

[dlyongemallo]

p.s. I just want to thank @jakelishman for his guidance in explaining the difference between how OpenQASM 2 and 3 represents the gates, which made this PR possible.