chrono: timescales and chronologies

The time::chrono module provides timescale utilities, and the foundations for
chronology with the [[moment]] type, an abstract, extendable temporal object.

Hare defines a chronology as a system for naming and ordering moments in time.
In practice, it is the combination of a calendar and wall clock. For the
Gregorian chronology, see the [[time::date::]] module.

The [[timescale]] interface facilitates leapsecond aware [[convert]]ion of
[[moment]]s. The [[tai]] timescale is the default intermediary timescale.
