Add Sum method and tests

[jreina]

https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.sum?view=netcore-2.1

[JamesMatchett]

Will take this one if that's okay? @jreina :)

[jreina]

It looks like there is already a PR in for this one, but I haven't gotten a chance to review it.

[JamesMatchett]

Grand no worries, will look for another issue I can do something for 👍 @jreina

[jreina]

Sounds good. There is also a JavaScript version of this repo
https://github.com/jreina/ShittyLINQ.js