[poincare] Clean Expression and Layout builders

Before: Opposite(Opposite(Rational(1))) misleadingly created the
expression "-1"; now, Opposite::Builder(Opposite::Builder(Rational(1)))
creates "--1"

apps/sequence/graph/term_sum_controller.cpp

@@ -50,9 +50,9 @@ double TermSumController::cursorNextStep(double x, int direction) {
 }
 
 Layout TermSumController::createFunctionLayout(const char * functionName) {
-  return HorizontalLayout(
+  return HorizontalLayout::Builder(
       CharLayout(functionName[0], KDFont::SmallFont),
-        VerticalOffsetLayout(
+        VerticalOffsetLayout::Builder(
           CharLayout('n', KDFont::SmallFont),
           VerticalOffsetLayoutNode::Type::Subscript
         )