contains_prefix
* implemented `contains_prefix` for StorageDoubleMap and StorageNMap Signed-off-by: muraca <mmuraca247@gmail.com> * match prefix to next_key Signed-off-by: muraca <mmuraca247@gmail.com> * warning unexpected behaviour with empty keys Signed-off-by: muraca <mmuraca247@gmail.com> * clarifications for unhashed::contains_prefixed_key Signed-off-by: muraca <mmuraca247@gmail.com> * added tests for StorageNMap Signed-off-by: muraca <mmuraca247@gmail.com> --------- Signed-off-by: muraca <mmuraca247@gmail.com>