Each thread has a stack of timers (which should have automatic scope.)  pushing a timer
onto the stack pauses the currently running one to avoid double counting, and popping a
timer resumes the previous one.

also make sure we pause the current timer, and swap out the current stack of timers when
swapping to/from generator stacks, since otherwise pushes/pops won't line up and we hit
asserts in StatTimer.

Timers and StatTimers accumulate their durations in terms of CPU ticks, which are converted
to microseconds only at Stats::dump() time, calculated with our estimate of CPU frequency.
Needless to say, this only works when there is no CPU stepping/scaling.

There are some additions to the Timer class that allow us to get the start/end time so that
we can sync up StatTimers with them, if both are used in a given piece of code.

getCPUTicks() is only implemented using __rdtscp() at the moment, so won't work on anything
not recent relatively recent x86.

we accumulate total ticks during dumping and output both 'ticks_in_main' and 'ticks_all_timers',
which should be equal.