the minimum and maximum for `DateZ` and `Duration` is now provided via
dedicated constants `{date,duration}::{MIN,MAX}`, much like built-in
`std::int` and others. they also now implements `std::num::Bounded`.
cf. rust-lang/rust#15934
- added a new example.
- reexported all public APIs in the crate root.
- made all constructors fail on the invalid arguments by default;
`*_opt()` variants have been added for the original behavior.
- same for `DateZ::{succ,pred}`.
- fixed a missing overflow check from `TimeZ::from_hms_{milli,micro}`.
Kang Seonghoon