Remove explicit default destructors (#157)

From CppCoreGuidelines:

Rule of Zero: C.20: If you can avoid defining default operations, do.
C.52: Use inheriting constructors to import constructors into a derived class that does not need further explicit initialization.
DRY forward and using declarations.
Miscellaneous:

Fix format.sh to output examples with normalised paths in sorted order.

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

src/ftxui/dom/flex.cpp

@@ -67,9 +67,8 @@ void function_not_flex(Requirement& r) {
 
 class Flex : public Node {
  public:
-  Flex(FlexFunction f) { f_ = f; }
+  Flex(FlexFunction f) : f_(f) {}
   Flex(FlexFunction f, Element child) : Node(unpack(std::move(child))), f_(f) {}
-  ~Flex() override {}
   void ComputeRequirement() override {
     requirement_.min_x = 0;
     requirement_.min_y = 0;