Draft: Get rid of deprecation warnings

[nordlow]

Gets rid of deprecation warnings via:

1. body => do
2. rename variable to avoid shadow warning
3. approxEqual => isClose: I started updating all those darn floating point literals to a better precision to fit isClose's requirements but I got fed up with it because it wasted a lot of time and I couldn't find a way to automated it. So I added an approxEqual that mimics old deprecated version to make tests pass without warnings nor unittest failures. But kept the constants refactorings. I can continue updating the rest of literals if needed but I'd rather not. :)
4. git ignore dstats-test-library

Ready for review now, @John-Colvin .

For reference see https://dlang.org/changelog/2.096.0.html#deprecate-approxEqual.