vis: adding a pure golang Vis()

The current Vis() and Unvis() are using the C implementation from
MTREE(8).

But that means that cgo is used, which is not always desired.

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

unvis.go

@@ -1,22 +1,7 @@
 package mtree
 
-// #include "vis.h"
-// #include <stdlib.h>
-import "C"
-import (
-	"fmt"
-	"unsafe"
-)
-
 // Unvis is a wrapper for the C implementation of unvis, which decodes a string
 // that potentially has characters that are encoded with Vis
 func Unvis(src string) (string, error) {
-	cDst, cSrc := C.CString(string(make([]byte, len(src)+1))), C.CString(src)
-	defer C.free(unsafe.Pointer(cDst))
-	defer C.free(unsafe.Pointer(cSrc))
-	ret := C.strunvis(cDst, cSrc)
-	if ret == -1 {
-		return "", fmt.Errorf("failed to decode: %q", src)
-	}
-	return C.GoString(cDst), nil
+	return unvis(src)
 }