Specialize formatter for all std::basic_string types (#3943)

* Specialize `formatter` for all `std::basic_string` types

* mock-allocator: add member types to make GCC 4.8 happy

test/mock-allocator.h

@@ -20,6 +20,16 @@ template <typename T> class mock_allocator {
   using value_type = T;
   using size_type = size_t;
 
+  using pointer = T*;
+  using const_pointer = const T*;
+  using reference = T&;
+  using const_reference = const T&;
+  using difference_type = ptrdiff_t;
+
+  template <typename U> struct rebind {
+    using other = mock_allocator<U>;
+  };
+
   mock_allocator() {}
   mock_allocator(const mock_allocator&) {}