Two small fixes for clang/Darwin build

On clang/Darwin, the array member `arr_` in StringLiteral was not
considered initalized when constructed using "". Adding default
initialization fixed the issue.

On clang/Darwin, std::string was not defined in Result.hpp. Including
<string> fixed the issue.

Author: melton1968

include/rfl/Result.hpp

@@ -5,6 +5,7 @@
 #include <ranges>
 #include <span>
 #include <stdexcept>
+#include <string>
 #include <tuple>
 #include <type_traits>
 #include <variant>

include/rfl/internal/StringLiteral.hpp

@@ -28,7 +28,7 @@ struct StringLiteral {
     return std::string_view(std::data(arr_), N - 1);
   }
 
-  std::array<char, N - 1> arr_;
+    std::array<char, N - 1> arr_{};
 };
 
 template <size_t N1, size_t N2>