Remove "=" & left-assoc opers from "OP" regex. Fixes #18.

Munging the right-associative "=" and "+", "-", etc., in with the "OP"
regex was creating issues where they were being treated as "list-intros"
by SMIE.

I have not quite figured out yet how SMIE determines whether something
is a "list-intro" token but this definitely fixes this issue.