merkle_tree: speed up iteration via next/1
merkle_tree: speed up iteration via next/1

-> instead of appending lists, allow a list of lists as the iterator type
jvf/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,jvf/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,jvf/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,jvf/scalaris,caijieming-baidu/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,jvf/scalaris,jvf/scalaris,jvf/scalaris,scalaris-team/scalaris