Add rules for marking result expressions.

[rusmonster]

While the problem of marking result expressions mentionered here still not solved in kotlin, it is good idea to fix it on the code style level due to avoid fights between developers :)

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[rusmonster]

I signed it!

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CLAs look good, thanks!

[JakeWharton]

From what I remember this is contrary to the advice of the JetBrains style guide for Kotlin. We'll have to make sure that's not the case, and even then i'm wary of such a hard rule for this.