Collection literals

This is an issue for discussion of **collection literals** proposal. The full text of the proposal is [here](https://github.com/Kotlin/KEEP/blob/bobko/collection-literals/proposals/collection-literals.md). Please, use this issue for the discussion on the substance of the proposal. For minor corrections to the text, please open comment directly in the PR https://github.com/Kotlin/KEEP/pull/417.