New features
+ Can now build a `Duration` by calling `.seconds()`, `.nanoseconds()`, etc. on several primitive types (unsigned and signed `int`s and `f32` and `f64`) (96).
Enhancements
+ Migrated the underlying computation from a generic lossless fraction to `TwoFloat`, a tuple of two 64bit floats to represent time. This led to an 85% reduction in computation time. (93)
+ Fix odd behavior of nanoseconds in string conversion (90)
+ Implement `FromStr` for `TimeSystem` for a cleaner API (95)
[Pull request here](https://github.com/nyx-space/hifitime/pull/99)