Add value_type, error_type typedefs to result. Refs #93.

include/boost/system/result.hpp

@@ -95,6 +95,11 @@ private:
 
     variant2::variant<T, E> v_;
 
+public:
+
+    using value_type = T;
+    using error_type = E;
+
 public:
 
     // constructors
@@ -444,6 +449,11 @@ private:
 
     variant2::variant<variant2::monostate, E> v_;
 
+public:
+
+    using value_type = void;
+    using error_type = E;
+
 public:
 
     // constructors