Simpler overloads that return tuples as default projections

[atifaziz]

I propose to add simpler overloads that return tuples as default projections where it makes most sense (even if it means something as simple as using ValueTuple.Create as the projector). In some cases, like Partition, we already do this. The idea would be to extend the following in a similar vein:

• Aggregate
• Cartesian
• CountDown
• Lead
• Lag
• Pairwise
• TagFirstLast
• EquiZip (Zip overload which returns sequence of tuples #558)
• ZipLongest (Zip overload which returns sequence of tuples #558)
• ZipShortest (Zip overload which returns sequence of tuples #558)

Did I miss any or does one not belong in the list above?

The result would be simpler usage, especially when using interactive and exploratory environments like Xamarin Workbooks, LINQPad, Try .NET, C# Interactive and others.

This is just a tracking issue. A separate PR should be open in each case.

[atifaziz]

I'm taking this up as no one has volunteered so far.

[atifaziz]

Added Aggregate, possibly including the experimental overloads, to the list.

[atifaziz]

@Orace I'd love a hand with this so thanks for your offer to help! You'll need to submit a PR to my fork against the vto branch and then I can integrate it into PR #598 post reviews.