Merge remote-tracking branch 'ztree/master'

1) All functions now starts with ztree- prefix
2) All files now have lexical-binding enabled

diff --cc packages/ztree/ztree-diff-model.el
Simple merge

diff --cc packages/ztree/ztree-diff.el
Simple merge

diff --cc packages/ztree/ztree-dir.el
Simple merge

diff --cc packages/ztree/ztree-util.el
Simple merge

diff --cc packages/ztree/ztree-view.el
Simple merge

diff --cc packages/ztree/ztree.el
Simple merge