ISO 8601 allows for both but prefers `,`, which was a main reason
for its use in rust-chrono. but this is too alien given other usages of
decimal points in Rust, so I guess it is better to switch to `.`.
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
Kang Seonghoon