message: Add constexpr constructor to StaticString
  Fixes a header usage problem for MSVC, which complains about StaticString not having a constexpr constructor thus making the "make" method not constexpr.
diff: @@ -77,7 +77,7 @@ public:
  using const_pointer = const value_type*;
  using const_iterator = const value_type*;
  - StaticString() noexcept = default;
  + constexpr StaticString() noexcept = default;
  // initialization from a string literal
  template <size_t N>
  