Make llcoro::scheduler log coros that run too long between yields.

Introduce LLCoros::CoroData::mHistogram, a map of cutoff times (bucket
breakpoints) with counts of occurrences. The idea is that mHistogram counts
how many times the real time taken by a particular coroutine resumption falls
into one of those buckets. Initialize the map with guessed buckets; these are
set in llcoros.cpp so they can be changed without requiring extensive rebuilds.

scheduler::pick_next() now records the timestamp and fiber context just before
the fiber manager resumes the next coroutine. If the next pick_next() call
reveals that the previous resumption took longer than the minimum bucket
breakpoint, it increments the appropriate bucket counter and logs the instance.

LLCoros::toplevel() reports nonzero mHistogram entries on coroutine
termination.

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