Quick fix for making LIKE into a comparison.  Probably should be handled
in the grouping filter instead.
Don't group NOT NULL as a single token (DRAGONS: this probably breaks DDL
fixes the expressions in SELECTs)
Make left-associative group work with AND / OR conditions

For now, the optional "assoc_left" param will keep any other grouping
methods from behaving differently, though it doesn't look like it's
strictly necessary.