Added a deconstructor for KeyValuePairs with comment

[Eregerog]

Implemented a nice deconstructor extension method for KeyValuePairs, which makes the following code possible:

foreach(var (key, value) in myDict) // ...

This is in a preprocessor directive, since the feature has no use before c#7. It would still compile, but probably just confuse people.

TODO for XDracam:

• Add a short mention in the Documentation, you can use a more fancy version of my example
• Check the preprocessor directive. I didn't get #if NET47 to work, so i had to use the UnityVersion one