vis: refactored code to reflect using vis/unvis for file names

Added some more test cases for `vis`ing and `unvis`ing
strings, and a test case that walks/checks a directory with
filenames that require encoding. Had to change Path() to account
for possible errors Unvis() could return. Refactored Vis()/Unvis() into
go-mtree tar functionality as well.

Signed-off-by: Stephen Chung <schung@redhat.com>

hierarchy.go

@@ -16,7 +16,8 @@ func (dh DirectoryHierarchy) WriteTo(w io.Writer) (n int64, err error) {
 	sort.Sort(byPos(dh.Entries))
 	var sum int64
 	for _, e := range dh.Entries {
-		i, err := io.WriteString(w, e.String()+"\n")
+		str := e.String()
+		i, err := io.WriteString(w, str+"\n")
 		if err != nil {
 			return sum, err
 		}