Add File.java to forbidden APIs

This commit cuts over all of core (not quite all tests) to java.nio.Path
It also adds the file class to the core forbidden APIs to prevent its usage
Unrelated to this change, but are we sure we need to follow links with this walkFileTree? This is a little complicated and scary, and the method has to track files it has seen and perform loop detection to try to make it safer. If we don't actually need to follow symlinks, then we dont have to worry as much.