Add left-assoc utility function and tests
Parenthesize multiple JOIN expressions

Microsoft Access complains about syntax errors when using
multiple joins.  The solution seems to be wrapping nested
joins in parentheses.

Example:

(use '[korma config core])
(defentity users)
(defentity emails)
(defentity phones)
(set-delimiters "[" "]")
(def q (-> (select* users)
         (join emails (= :users.email :email.addr))
         (join phones (= :users.phone :phone.num))))
(sql-only (select q))

SELECT [users].*
FROM ([users]
      LEFT JOIN [emails] ON [users].[email] = [email].[addr])
      LEFT JOIN [phones] ON [users].[phone] = [phone].[num]