Call Stack Samples to Events

A sampling profiler records samples, a list of [timestamp, stack] pairs in increasing timestamp order. stack lists the active function names from outermost to innermost. Return the call events as strings "start <name> <t>" and "end <name> <t>".

The longest common prefix of two consecutive stacks keeps running. At the later timestamp, the remaining frames of the earlier stack end, innermost first, and then the remaining frames of the later stack start, outermost first. The first sample starts all of its frames. After the last sample, every open frame ends at its timestamp, innermost first.