Added untested method to drop back to kernel mode.

May require kernel mode to be user-accessible, which is not ideal. If so, kill
this and don't try to return once user mode is entered.

Part of the fix for issue #12.
Enable interrupts for user mode
Adding string::find and a bunch of string tests. Also string fixes.
Adding test to presubmit and fixing some issues
Adding tests for ARRAY_SIZE.

Fixes issue #7.