Avoid unnecessary copies in generated VisitorTransforms (#75)

* Avoid uneccessary copies in generated VisitorTransforms

Previously, the generated VisitorTransform classes *always* made a
deep copy of the input tree. With this commit, only those nodes which
are actually changed will be copied.

In the transform*() functions of the VisitorTransform super classes,
we now check to see if all of the new child nodes are the same
instance as the original child nodes.  If so, there is no need to
copy the original node.

Note that this does not apply to cross-domain visitor transforms
since the type of the nodes is changing, it is still neccessary to
perform the deep copy.

pig-runtime/src/org/partiql/pig/runtime/DomainVisitorTransformBase.kt

@@ -26,23 +26,38 @@ import com.amazon.ionelement.api.MetaContainer
 abstract class DomainVisitorTransformBase {
     open fun transformMetas(metas: MetaContainer) = metas
 
-    open fun transformAnyElement(node: AnyElement): AnyElement =
-        // TODO:  remove .asAnyElement() below when https://github.com/amzn/ion-element-kotlin/issues/36 is fixed.
-        node.copy(metas = transformMetas(node.metas)).asAnyElement()
+    open fun transformAnyElement(node: AnyElement): AnyElement {
+        val newMetas = transformMetas(node.metas)
+        return if(node.metas !== newMetas) {
+            node.copy(metas = newMetas).asAnyElement()
+        } else {
+            node
+        }
+    }
 
-    open fun transformSymbolPrimitive(sym: SymbolPrimitive) =
-        SymbolPrimitive(
-            transformSymbolPrimitiveText(sym),
-            transformSymbolPrimitiveMetas(sym))
+    open fun transformSymbolPrimitive(sym: SymbolPrimitive): SymbolPrimitive {
+        val newText = transformSymbolPrimitiveText(sym)
+        val newMetas = transformSymbolPrimitiveMetas(sym)
+        return if(sym.text != newText || sym.metas !== newMetas) {
+            SymbolPrimitive(newText, newMetas)
+        } else {
+            sym
+        }
+    }
 
     open fun transformSymbolPrimitiveText(sym: SymbolPrimitive) = sym.text
 
     open fun transformSymbolPrimitiveMetas(sym: SymbolPrimitive) = transformMetas(sym.metas)
 
-    open fun transformLongPrimitive(lng: LongPrimitive) =
-        LongPrimitive(
-            transformLongPrimitiveValue(lng),
-            transformLongPrimitiveMetas(lng))
+    open fun transformLongPrimitive(lng: LongPrimitive): LongPrimitive {
+        val newValue = transformLongPrimitiveValue(lng)
+        val newMetas = transformLongPrimitiveMetas(lng)
+        return if(lng.value != newValue || lng.metas !== newMetas) {
+            LongPrimitive(newValue, newMetas)
+        } else {
+            lng
+        }
+    }
 
     open fun transformLongPrimitiveValue(sym: LongPrimitive): Long = sym.value