Allow VALUEs to be enclosed in double and single quotes, specifically
was a problem on ENUMs, so I added a test for this.
fix diff for altering two things per column - add ; at the end

replaced /;$/xms with /;\s*\z/xms since alter_field can return
two or more statements
produce_diff_sql(): list context

rather change the calling of alter_field and the other methods to list context
so that if they return multiple statements produce_diff_sql() can return
them also seperately in list context