Improve DateTime comparisons

[Exadra37]

@Qqwy

There has been significant discussion about this in the past on the mailing list.
There were indeed people who liked using :<, := and :> as comparison results, but in the end the core team decided against it.

I am now curious to know why the explicit semantics were not preferred

@Qqwy

As for the helper functions you propose: DateTime was included into Elixir’s standard library mainly to have a single datatype that the different date/time-libraries would agree on, rather than each library ending up with its own incompatible types.

I have not proposed them, but @gregvaughn was who provided them based on the @Awlexus suggestions