fix timestamp const function

include/sled/status_or.h

@@ -28,7 +28,7 @@ public:
 
     StatusOr(StatusOr &&other)
         : status_(std::move(other.status_)),
-          value_(std::move(value_))
+          value_(std::move(other.value_))
     {
         other.status_ = MakeDefaultStatus();
     }

include/sled/units/timestamp.h

@@ -63,7 +63,7 @@ public:
     }
 
     template<typename T = int64_t>
-    constexpr T ns()
+    constexpr T ns() const
     {
         return ToMultiple<1000, T>();
     }