Add ripple::Expected which simulates std::expected:

Also integrates use of ripple::Expected into the code base.

src/ripple/protocol/TER.h

@@ -349,7 +349,10 @@ public:
     }
 
     // Trait tells enable_if which types are allowed for construction.
-    template <typename T, typename = std::enable_if_t<Trait<T>::value>>
+    template <
+        typename T,
+        typename = std::enable_if_t<
+            Trait<std::remove_cv_t<std::remove_reference_t<T>>>::value>>
     constexpr TERSubset(T rhs) : code_(TERtoInt(rhs))
     {
     }