This was disabled explicitly in recent commit
232c948674cff1b4b72c2d6b3acfa7342d096dd1 to prevent using
the <compare> header from STL.
Now, re-enabling by implementing an ETL version of it in compare.h
* Fix & add more tests for year_month arithmetic
* Minor addtions to previous commit
* More missing values to be uninitialized
* Update the default constructors to = default and correct default constructor tests accordingly
* Fix & add more tests for year_month arithmetic
* Minor addtions to previous commit
* More missing values to be uninitialized
* Update the default constructors to = default and correct default constructor tests accordingly
* Restore default constructor behavior for chrono calender