[Breaking] changes for HolidayCalendarCache

[felipenoris]

Inspired by the job done at https://github.com/felipenoris/bdays, I'm planning a major breaking change for how cache works in this package.

Planned changes:

• BusinessDays.initcache will be removed

• BusinessDays.HolidayCalendarCache will be a subtype of BusinessDays.HolidayCalendar

Collateral effects:

• These changes imply that bdays(:calendar_symbol, d0, d1 method won't work for cached calendars anymore. The user will need to pass an instance of HolidayCalendarCache.

• internal package state variable BusinessDays. CACHE_DICT will be removed.

After this change, I'll tag a v0.10.0 transition version with deprecation warnings, and then I'll release a v1.0.0, in compliance with Semantic Versioning .