Remove comment stack from the future parser's lexer

The lexer tracks comments in a stack. This code is error prone and broke when
introducing new language elements. For now we're going to remove it and only
use the current parser to get doc information.
Fix test for future parser in arithmetic operators

When checking whether concatenation is supported the check for the
Puppet[:parser] setting was wrong (checking for a symbol instead of array).
This caused concatenation to not be supported even with the future parser
enabled.