Unnecessary dependency on chrono

[lffg]

First of all, thanks for your work on this library :-)

At $work, our services depend on the time crate for date/time stuff. There is also the new jiff crate, which will probably gain some popularity in the future. In general, since the Rust ecosystem has many date/time libraries, one could argue that libraries such as bdays should be agnostic over them.

However, that's currently not the case, as the HolidayCalendar trait has a bound restricting the generic type to the chrono Datelike trait. As a solution, bdays could define and expose its own Datelike trait. WDYT?

I may be able to send a PR.