WIP: Added equality and comparison to ParseResults<_>.

[kleidemos]

I had problem when I added a subcommand case to Arguments. Because DU containing ParseResults<_> loses structural equality and comparison. I think comparing a ParseResults<_> by GetAllResults is a good idea.

• Safer Code: Declaring Conditions for Equality over Container Types in https://blogs.msdn.microsoft.com/dsyme/2009/11/08/equality-and-comparison-constraints-in-f/

[eiriktsarpalis]

Thanks for the PR, but I think I won't be merging this for the following reasons:

• It adds a breaking change by introducing additional constraints to the generic parameter.
• The changes mean .GetAllResults() would be getting called every time a comparison is made.
• There is an obvious workaround here, namely just manually calling .GetAllResults() manually and comparing the results.

[kleidemos]

• It does not introduce additional constraints. It adds structural equality and comparison if the generic paramter has them. Like obj list and int list.

  With these attributes, MyBox<A> will only satisfy the equality and comparison constraints if A satisfies these constraints. You will still be able to use MyBox<_> with any type.

• At the moment nobody uses comparison, because ParseResults<_> is not comparable. In any case we can make it lazy.
• When I compare Arugments with ParseResults<_> (even in only one case), I must manually write comparison for all cases.

[eiriktsarpalis]

Fair enough. Can revisit if you could cache the result for comparison purposes and add a few tests.

[eiriktsarpalis]

Thanks