[Proposal] Add support for traits and/or mixins

[dsaf]

Description:

http://en.wikipedia.org/wiki/Trait_%28computer_programming%29

Previous discussions:

https://roslyn.codeplex.com/discussions/570952
https://roslyn.codeplex.com/discussions/542646

[gafter]

@dsaf As far as I can tell #129 has nothing to do with this. #129 is about constraints, and this is about behavioral inheritance.

[dsaf]

@gafter you are right.