Add isless(::Nullable, ::Nullable) -> Bool

[nalimilan]

This method complements isequal(), which also returns a Bool, as it
must implement a total order. NULLs are treated like NaNs: they are
never isless() than any other value, including other NULLs. They are
therefore always sorted last.

(Travis failure on master is unrelated.)

[davidagold]

This seems reasonable. But do you have a sense of how many more of these "special cases" (i.e., functions on Nullables that do not return Nullable) are needed?

[nalimilan]

Very few AFAICT. Looks like we all agree that all operators should return a Nullable, except for isequal and isless, and comparison operators (==, <, >) for which there's no consensus yet. So after this PR (which I'll open against Julia Base too) it should be complete.

[davidagold]

Yeah, I'd prefer that this live in Base. Could you please x-ref that PR when you open it?

[nalimilan]

I'd still like this to be included in NullableArrays too so that it works on 0.4 and 0.5. Requiring 0.6 is quite cumbersome at this point.

[davidagold]

Fair, but can we organize the files so that this method can be grouped amongst those that should really live somewhere else?

[nalimilan]

I think that's the case. Aren't Nullable operators supposed to go into Base or into a separate package at some point? Also, this definition is very dependent on the infrastructure used for other operators.

[davidagold]

Sounds good to me for now.

[nalimilan]

Filed PR against Base: JuliaLang/julia#18304