Reorder Kernel methods
Add #taint placeholders for compatibility
Avoid expand_path in require_relative

The problem is that at compiler level there’s no reference to the file
on disk or to its path. This makes require_relative a bit less correct
but fixes it for most cases.

Fixes #626
Raise NotImplementedError in String mutable meths

This is for improved usability, previously method_missing was called.
WIP
[snapshot]