Time and RTC changes

CosaBenchmarkRTCTimer won't compile: needs `micros` mutator in RTC.

Modified `RTC::seconds` so that it restarts the ticks and error part of the second.  This is probably the intent of `seconds` mutator anyway: *NOW* it's this second.  Otherwise, it could unexpectedly rollover into the next second shortly after the call, or skip a tick because the error had been accumulating for a while.

Minor changes to `time_t` to allow streaming `const time_t` and packing in enclosing structures/classes.