Improve DateTime comparisons

[Exadra37]

@gregvaughn

Those all seem like one-liners around https://hexdocs.pm/elixir/1.10.4/DateTime.html#compare/2

def my_compare(a, :<, b), do: DateTime.compare(a, b) == :lt
def my_compare(a, :<=, b), do: DateTime.compare(a, b) != :gt

In my opinion this is a much better API then the core implementation, but maybe is only me that doesn’t like :lt, :gt and friends