tokenize-786661: Minor comment update

Signed-off-by: Joe Walker <jwalker@mozilla.com>
tokenize-786661: Fix handling of \\n

We were previously making errors due to the double parsing that we were
doing, rather than write our own code to step through we just do the
replacement in 2 steps. See code comments for details.

Signed-off-by: Joe Walker <jwalker@mozilla.com>