✨ add ApproximateDateTimeEqualityComparer, to be used in deep equalit…

…y testing

GetHashCode won't return equal values for approximately-equal inputs, unfortunately
- still need to have a think on this, but I don't think it's doable: imagine:

we have 3 values: A, B, C
we allow a 1s drift
B - A = 750ms
C - B = 750ms

A ~ B (within 1s)
C ~ B (within 1s)
A !~ C (1.5s apart)