Make `TimeSpan` a subtype of `AbstractInterval`

[omus]

A good intermediate step to switching over to Intervals.jl completely. Makes the TimeSpan type a subtype of AbstractInterval{Nanosecond,Closed,Open} and implements functionality using Intervals functions.

Main things to do here:

• [ ] Update generic AbstractInterval functions to return TimeSpan instead of Interval. Changing this will probably require some formalizing of the intervals interface
• [ ] Add deprecations for no longer needed TimeSpan functions
• [ ] Upstream changes that belong elsewhere

[haberdashPI]

Is this something I can help move things forward on. It would be nice if the work I put into https://github.com/invenia/Intervals.jl/pull/192 would work seamlessly with TimeSpans.