Fix for buggy Base.isless impl

[torfjelde]

Currenty we have

julia> LogDensityProblems.LogDensityOrder{0}() < LogDensityProblems.LogDensityOrder{0}()
true

which seems incorrect, no?

With this PR:

julia> LogDensityProblems.LogDensityOrder{0}() < LogDensityProblems.LogDensityOrder{0}()
false

[devmotion]

Yes, this is a bug. From the docs of isless:

if it is defined, it is expected to satisfy the following:

- If isless(x, y) is defined, then so is isless(y, x) and isequal(x, y), and exactly one of those three yields true.
- The relation defined by isless is transitive, i.e., isless(x, y) && isless(y, z) implies isless(x, z).

[devmotion]

Thank you!