wall clock timing functionality

This is probably fairly low priority, but: an interesting and sometimes-useful feature is the ability to sleep until a particular wall clock time (e.g., "this cert is expiring at 2017-12-31T12:00:00Z, so I want to wake up 3 days before that so I can renew it"). This is quite different from trio's current timekeeping abilities, which are all oriented around monotonic time (which ignores clock changes, and stops while the computer is suspended, etc.). Really these are just different incommensurable time scales.

Fortunately, I don't think we need the ability to directly set a cancel scope deadline to a particular wall clock time, so this doesn't need to be integrated deep into the guts of trio's run loop. I think it'd be sufficient to provide a sleep_until_wall_clock_time (and maybe current_wall_clock_time for completeness).

This comment and the replies have some more details on this idea, including notes on how it could be implemented on different systems.

Read Trio's existing monotonic timekeeping APIs first, then follow the linked comment and replies on issue #168 for platform-specific implementation details. Done means providing the proposed wall-clock sleep capability, optionally with a current-wall-clock-time helper, without integrating wall-clock deadlines into the run loop.