Implement a poller

I believe this is a correct implementation, but there are some edge-cases that still need to be handled (see the TODOs). @halleysfifthinc, this could be used for the async comms stuff by having a poller waiting on the kernel socket *and* an internal socket to which we send messages to be forwarded to the kernel socket. CC @CasBex Fixes #52. EDIT: ok, maybe not so correct on windows according to CI.