Remove `U` macro definition from public headers

[aminya]

Cpprest does some questionable macro definitions here.
microsoft/cpprestsdk@411a109/Release/include/cpprest/details/basic_types.h#L87

The U Macro overrides the U typename in fmt causing fmt not to work. This definition is like a time-bomb waiting to cause problems anytime.

  template <typename T, typename U = format_as_t<T>,
            FMT_ENABLE_IF(std::is_arithmetic<U>::value)>
  FMT_MAP_API auto map(const T& val) -> decltype(FMT_DECLTYPE_THIS map(U())) {
    return map(format_as(val));
  }

Related fmtlib/fmt#4180

Can you remove this macro?