Added performance counters to zippers

[imlvts]

This introduces timed_span macros used in all the zipper methods.
The macro uses a global array for each span, incrementing times the span was entered, and platform-specific TSC delta spent in the span.
The counters may be either read from a static array, or printed using pathmap::timed_span::print_counters();.

[luketpeterson]

Hey. Sorry I didn't see this PR amid all the other churn.

High-level questions:

• Is this feature intended for internal use (optimizing pathmap) or external use (helping users optimize code that uses pathmap)?
• Is the cost negligible to have it always on? Or should be be behind a feature flag when it's not needed?
• Is it going to cause a problem that some zipper implementations provide their own implementations of some methods, and therefore won't call the default impls with the timed_span! macro invocations in them?

[Adam-Vandervorst]

Can we pull this at least into a non master branch @luketpeterson ?

[Adam-Vandervorst]

Bump @luketpeterson

[luketpeterson]

I have no problem main-lining this, but if it's targeted towards internal optimization then it probably belongs behind a feature gate.

@imlvts, What are your opinions on these questions? #14 (comment)