vis/unvis: pull in exact implementation from FreeBSD

Perhaps this is not completely ideal, because it brings in cgo.
And with the flags, it can have tailored experience.

I've added a basic test to ensure that the cases we're interested in are
covered.

This does not yet integrate the usage of Vis()/Unviz() into the manifest
create and compare.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

unvis.go

@@ -0,0 +1,15 @@
+package mtree
+
+// #include "vis.h"
+import "C"
+import "fmt"
+
+func Unvis(str string) (string, error) {
+	dst := new(C.char)
+	ret := C.strunvis(dst, C.CString(str))
+	if ret == 0 {
+		return "", fmt.Errorf("failed to encode string")
+	}
+
+	return C.GoString(dst), nil
+}