Nesting profilings

[glensc]

This is perhaps a discussion for collector part, but I'll discuss it here to have better visibility and may involve xhgui itself too.

I realized that all profiles we support, do not support nesting, but probably it's possible to emulate nesting by merging the profiling data or extracting a subset of data in userspace.

[markstory]

What would create a nested profile?

[glensc]

With given example:

1. $profiler->start();
2.   $profiler->start();
3.   $profiler->stop();
4. $profiler->stop();

in my code, I have overall profiler running (1), now I want to profile subset of code (2-3), but that should not interfere with the overall profiling results (4).