This adds a simple timer that can be used for performance measurements in C++.
More sophisticated timers are possible (e.g. one that takes averages, logs the results, ...).
However, I found that this simple timer is good enough for 99% of my use cases.
To use it just write `SCOPED_TIMER("my timer name");` or more commonly `SCOPED_TIMER(__func__);`
into some scope.
Reviewers: sergey
Differential Revision: https://developer.blender.org/D7491
