SQL-OPERATION — Constructs an SQL expression from a supplied operator and arguments.Function
      sql-operation operator &rest args => result
      sql-operation 'function func &rest args => resultoperatorA symbol denoting an SQL operator.
funcA string denoting an SQL function.
argsA set of arguments for the specified SQL operator or function.
A object of type sql-expression.
Returns an SQL expression constructed from the supplied
      SQL operator or function operator and
      its arguments args. If
      operator is passed the symbol 'function
      then the first value in args is taken to
      be a valid SQL function and the remaining values in
      args its arguments.
      
(sql-operation 'select 
          (sql-expression :table 'foo :attribute 'bar)
          (sql-operation 'sum  (sql-expression :table 'foo :attribute 'baz))
          :from 
          (sql-expression :table 'foo) 
          :where 
          (sql-operation '> (sql-expression :attribute 'bar) 12)
          :order-by (sql-operation 'sum (sql-expression :attribute 'baz)))
=> #<SQL-QUERY SELECT FOO.BAR,SUM(FOO.BAZ) FROM FOO WHERE (BAR > 12) ORDER BY SUM(BAZ)>
(sql-operation 'function "strpos" "CLSQL" "SQL")
=> #<CLSQL-SYS:SQL-FUNCTION-EXP STRPOS('CLSQL','SQL')>
      An error of type sql-user-error
      is signalled if operator is not a symbol
      representing a supported SQL operator.