Improve STVector256 deserialization

src/ripple/basics/base_uint.h

@@ -26,6 +26,7 @@
 #define RIPPLE_BASICS_BASE_UINT_H_INCLUDED
 
 #include <ripple/basics/Expected.h>
+#include <ripple/basics/Slice.h>
 #include <ripple/basics/contract.h>
 #include <ripple/basics/hardened_hash.h>
 #include <ripple/basics/strHex.h>
@@ -56,6 +57,11 @@ struct is_contiguous_container<
 {
 };
 
+template <>
+struct is_contiguous_container<Slice> : std::true_type
+{
+};
+
 }  // namespace detail
 
 /** Integers of any length that is a multiple of 32-bits