closes #29 closes #14

TODO

[x] cpu_bound and io_bound
[x] Effect.__repr__
[x] handle extension types in docs
[x] how to distribute c files?
[x] wheels on circleci?
[x] what to do about generic extension types? Just explain to use __future__.annotations?
[x] include custom pytkdocs for building docs on rtfd.io
[x] test stack safety of effects that call resume in apply_continuation
[x] test repr
[x] improve effect hypothesis strategy

suned / pfun