add SX and SDG gates

[kevinsung]

After the quimb release that includes jcmgray/quimb#277

TODO:

• Also bump minimum quimb version

[garrison]

It will also be necessary to bump the minimum-required version of quimb, along with this current change. (Actually, we should set up some minimum version tests, which would lead to CI failing if we forgot to do this.)

[kevinsung]

Actually, we should set up some minimum version tests, which would lead to CI failing if we forgot to do this.

Good idea, I opened #22 .