Rewrite time module in pure Rust

[jbangelo]

The next chunk of changes from #122.

This re-implements the time related functionality in native Rust. This one is a bit larger, mostly from the fact that the conversions in the C implementation are very inter-dependent and so several new bits of functionality were pulled in to make the Rust implementation mirror the C implementation. There's a few bits of functionality thrown out because they caused some significant unsoundness, but I'm sure there is still several bits of unsoundness in this translation (e.g. several panics/asserts are still present). I'd prefer to keep this PR as close to a straight translation as possible, and re-consider the soundness concerns in a future PR.

What's been changed

• I don't think any implementations are still present but meaningfully modified

What's been removed

• GloTime - GLONASS time is particularly complicated and rarely used
• GpsTime::to_glo() and GpsTime::to_glo_hardcoded() - See above
• impl Sub<GpsTime> for GpsTime - It turns out a std::time::Duration can't hold a negative value. This implementation would occasionally panic because of this. The chrono crate handles this better, maybe we can move towards their approach in a later PR?
• UtcParams::decode() - Decoding low-level messages seem unlikely to be needed
• impl Default for UtcParams - Defaulting this data can cause unexpected errors
• impl Default for UtcTime - Defaulting this type can cause unexpected errors

What's been added

• The time::consts module
• GpsTime::from_date() and GpsTime::from_date_hardcoded()
• GpsTime::to_mjd() and GpsTime::to_mjd_hardcoded()
• GpsTime::to_date() and GpsTime::to_date_hardcoded()
• UtcTime::to_date()
• MJD::to_gps() and MJD::to_gps_hardcoded()
• MJD::to_date()