Add TreeModel and HasSubtree implementation

src/orm/builder/AbstractBuilder.ts

@@ -222,17 +222,45 @@ export abstract class AbstractBuilder<T> extends AppClass {
         return this
     }
 
+    /** Apply a WHERE ... IS NULL constraint to the query. */
+    whereNull(field: string): this {
+        return this.whereRawValue(field, 'IS', 'NULL')
+    }
+
+    /** Apply a WHERE ... IS NOT NULL constraint to the query. */
+    whereNotNull(field: string): this {
+        return this.whereRawValue(field, 'IS NOT', 'NULL')
+    }
+
     /**
      * Apply a new WHERE constraint to the query, without escaping `operand`. Prefer `where()`.
      * @param field
      * @param operator
      * @param operand
      */
-    whereRaw(field: string, operator: ConstraintOperator, operand: string): this {
+    whereRawValue(field: string, operator: ConstraintOperator, operand: string): this {
         this.createConstraint('AND', field, operator, raw(operand))
         return this
     }
 
+    /**
+     * Add raw SQL as a constraint to the query.
+     * @param clause
+     */
+    whereRaw(clause: string|QuerySafeValue): this {
+        if ( !(clause instanceof QuerySafeValue) ) {
+            clause = raw(clause)
+        }
+
+        this.constraints.push(raw(clause))
+        return this
+    }
+
+    /** Apply an impossible constraint to the query, causing it to match 0 rows. */
+    whereMatchNone(): this {
+        return this.whereRaw('1=0')
+    }
+
     /**
      * Apply a new WHERE NOT constraint to the query.
      * @param field