Compact representation for a calendar

This modules basically builds a data-structure for a set of days based on bit-maps. This is built to store a collection of regional holidays for the opening-hours crate.

Data layout

Here is how serialized data is represented:

 start   size       year 1          year 2      ...
┌──────┬──────┬───────────────┬───────────────┬────
│  8B  │  8B  │ 8B * 12 = 96B │ 8B * 12 = 96B │ ...
└──────┴──────┴───────────────┴───────────────┴────

Each year is just an array of 12 u32 where the least significant bits each represent a day.

While a bitset might not be the most efficient way to store a collection of dates for sparse data, this approached proved to be very compact when combined with a Zlib encoder. This methods allowed to store all holidays from 2000 to 2100 as described by workalendar in only 60kb of data.