Allow component re-use

Partial fix for hexops#78, only allows re-use of *vecty.Component, not
*vecty.HTML

dom.go

@@ -167,11 +167,11 @@ func (h *HTML) restoreHTML(prev *HTML) {
 			continue
 		}
 		prevChild := prev.children[i]
-		prevChildRender, ok := prevChild.(*HTML)
-		if !ok {
+		prevChildRender, isHTML := prevChild.(*HTML)
+		if !isHTML {
 			prevChildRender = prevChild.(Component).Context().prevRender
 		}
-		if nextChildRender == prevChildRender {
+		if nextChildRender == prevChildRender && (isHTML || prevChild != nextChild) {
 			panic("vecty: next child render must not equal previous child render (did the child Render illegally return a stored render variable?)")
 		}
 		if doRestore(prevChild, nextChild, prevChildRender, nextChildRender) {
@@ -207,7 +207,7 @@ func (h *HTML) Restore(old ComponentOrHTML) {
 		}
 	}
 
-	if prev, ok := old.(*HTML); ok && prev != nil {
+	if prev, ok := old.(*HTML); ok && prev != nil && prev.Node != nil {
 		if h.text != "" && prev.text != "" {
 			h.restoreText(prev)
 			return