IsNull should also be fluent.

Also we should not return In or IsNull from our FluentExp methods
because those are impl classes and expose no public API.

src/main/java/com/samskivert/depot/expression/FluentExp.java

@@ -59,19 +59,19 @@ public abstract class FluentExp<T>
     }
 
     /** Returns an {@link IsNull} with this expression as its target. */
-    public IsNull isNull ()
+    public FluentExp<Boolean> isNull ()
     {
         return new IsNull(this);
     }
 
     /** Returns an {@link In} with this expression and the supplied values. */
-    public In in (Comparable<?>... values)
+    public FluentExp<Boolean> in (Comparable<?>... values)
     {
         return new In(this, values);
     }
 
     /** Returns an {@link In} with this column and the supplied values. */
-    public In in (Iterable<? extends Comparable<?>> values)
+    public FluentExp<Boolean> in (Iterable<? extends Comparable<?>> values)
     {
         return new In(this, values);
     }

src/main/java/com/samskivert/depot/impl/operator/IsNull.java

@@ -7,14 +7,14 @@ package com.samskivert.depot.impl.operator;
 import java.util.Collection;
 
 import com.samskivert.depot.PersistentRecord;
+import com.samskivert.depot.expression.FluentExp;
 import com.samskivert.depot.expression.SQLExpression;
 import com.samskivert.depot.impl.FragmentVisitor;
 
 /**
  * The SQL 'is null' operator.
  */
-public class IsNull
-    implements SQLExpression<Boolean>
+public class IsNull extends FluentExp<Boolean>
 {
     public IsNull (SQLExpression<?> expression)
     {