Enum constant names

[yole]

The style guide doesn't talk about naming enum constants. They don't fall under "Constant values can only be defined inside of an object or as a top-level declaration", so they are not covered by that rule, and there should be a separate one for them.

(And please don't require UPPER_CASE for all enum constants; that would be ugly. Our style guide allows both UPPER_CASE and CamelCase.)

[hho]

@yole Please consider updating Kotlin's enum reference page to reflect that. Currently it looks like UPPER_CASE is the way to go.

(which I liked, because it's good for Java interop)