chrono-intervals: Grouped time intervals for Rust

Create chrono time intervals as "per-day", "per-week" etc.

Defaults and options

• Only timezone-aware Datetime objects can be passed in and retrieved.
• All retrieved intervals are in the UTC timezone.
• By default, the first and last interval are extended to enclose the specified beginning and ending. Both extensions can be switched off separately with options.
• By default, the end of an interval is 1ms before the start of the next interval. The precision of this can be specified with options.