Kotlin's default visibility should be internal

Public vs. private stats for Ceylon SDK and IDE (plus some opinions): https://twitter.com/1ovthafew/status/672803401990848513

For application code, public vs. internal doesn’t really matter. For library code, defaulting to public seems odd.

It would be good to have a well thought out visibility story for Kotlin + JDK9/Jigsaw (which defaults packages to internal).