This change fixes the edge case where a negative sleep time is provided.
When this happens, the call now returns immediately (as specified by the docs for time.Sleep).
This commit refactors both determining the current time and sleeping for
a given time. It also improves precision for many chips.
* The nrf chips had a long-standing TODO comment about a slightly
inaccurate clock. This should now be fixed.
* The SAM D2x/D5x chips may have a slightly more accurate clock,
although probably within the error margin of the RTC. Also, by
working with RTC ticks and converting in the least number of places,
code size is often slightly reduced (usually just a few bytes, up to
around 1kB in some cases).
* I believe the HiFive1 rev B timer was slightly wrong (32768Hz vs
30517.6Hz). Because the datasheet says the clock runs at 32768Hz,
I've used the same conversion code here as in the nrf and sam cases.
* I couldn't test both stm32 timers, so I kept them as they currently
are. It may be possible to make them more efficient by using the
native tick frequency instead of using microseconds everywhere.
