So far there was an implicit assumption hard-coded that collections are
always lists. However, sets are also collections, and can be serialized
to JSON arrays just like lists.

This change allows to serialize generic collections independently of the
concrete implementation.

Fixes Kotlin#1421.

Now with `CollectionSerializer` inheriting from `ListLikeSerializer`, it
makes sense to rename `ListLikeSerializer` to the more generic
`CollectionLikeSerializer`.

Done by running `./gradlew apiDump`.