do we need to store the key twice? I think the "key.toString()" could be replaced by something like "null", "true", "0" to save some time. Or we could do something like
```
Dict.prototype.binarySet = function binarySet(key, value) {
   var hexkey = key.toString("hex")
   this._keys[hexkey] = hexkey
   this[key] = value
 }
```
OR
```
Dict.prototype.binarySet = function binarySet(key, value) {
   var strkey = key.toString()
   this._keys[key.toString('hex')] = strkey
   this[strkey] = value
 }
```

Last one seems to be the fastest, even if I don't understand why this[key.toString()] can be faster then this[key]. Node should use toString() internaly to convert the buffer to a string, shouldn't it?