Shared memory array API and optional tight integration with `numpy`

[goodboy]

Initial module import from piker.data._sharemem and first implementation of the system(s) discussed in #339.

---

Would like to have done for this to land:

• [ ] test(s) which reproduces https://github.com/pikers/piker/issues/306 and https://github.com/pikers/piker/issues/332
  • [ ] ideally we also can start running CI against OS X as in #255
• [x] naive support for a readonly capable shared list wrapper around ShareableList
• [ ] code adjustements for optional numpy.ndarray support
  • [ ] use nptyping to get mypy happy (as possible)
• [x] first example of a readers-write lock style with both numpy and ShmList type
  • [x] include in test suite
  • [x] add a "frame size" parametrization
  • [ ] document this in the readme?

[jaredgoldman]

See issue regarding OSX support here: https://github.com/goodboy/tractor/issues/340