Migrate to hifitime for time keeping

[gsleap]

From Chirs Jordan: "I'm not sure how familiar you are with the rust library hifitime. We're using it in Marlu (and onwards) and it's wonderful; you can represent timestamps with nanosecond precision (hifitime::Epoch) and the struct methods allow you to convert to whatever (GPS, MJD, JD etc.). So, it might make sense to eventually use this as the single time-keeping method in the mwalib context structs, rather than having multiple time formats and things like seconds and milliseconds. Time resolutions etc. can be represented by hifitime::Duration, and this has the advantage of tracking units internally. I guess the trickiest part would be FFI; it's probably not worth exposing hifitime types to FFI callers, but you could convert to whatever you needed at the boundary. Just some food for thought."