Some summary of changes:
- Don't use DEG prefix for types and enumerator values:
the code is already inside DEG namespace.
- Put code where it locally belongs to: avoid having one
single header file with all sort of definitions in it.
- Take advantage of modern C++11 enabled by default.
This statistics is only collected when debug_value is different from 0.
Stored in depsgraph node itself, so we can always have access to average data
and other stats which requires persistent storage. This way we also don't waste
time trying to find stats from a separately stored hash map.
