streams: Add SpanReader ignore method

Needed to deserialize some types from spans like CScripts

src/streams.h

@@ -182,6 +182,11 @@ public:
         memcpy(dst.data(), m_data.data(), dst.size());
         m_data = m_data.subspan(dst.size());
     }
+
+    void ignore(size_t n)
+    {
+        m_data = m_data.subspan(n);
+    }
 };
 
 /** Double ended buffer combining vector and stream-like interfaces.