Make the creation of true and false literals less hacky.

src/main/java/com/samskivert/depot/Exps.java

@@ -39,6 +39,22 @@ public class Exps
         return new ValueExp(value);
     }
 
+    /**
+     * Creates an expression that evaluates to true.
+     */
+    public static SQLExpression trueLiteral ()
+    {
+        return literal("true");
+    }
+
+    /**
+     * Creates an expression that evaluates to false.
+     */
+    public static SQLExpression falseLiteral ()
+    {
+        return literal("false");
+    }
+
     /**
      * Creates a literal expression with the supplied SQL snippet. Note: you're probably breaking
      * cross platform compatibility by using this construction.