Implement dict to receive Object as key, not only String

https://github.com/go-python/gpython/blob/33327c5231b03c84972bec57262694aebbaa1e84/py/dict.go#L68

Change the dict to receive a hashable object as a key, not just a string.
I will change it so that the object can be looked up through the hash value of the object.

Store the object in the slice and use the map to find the index of the stored object through the hash value.
```go
make([]Object, 0, len(default_size)) // slice to store object (index -> Object)
type Dict map[HashIndex]int // map to store index of slice (Hash -> index)
```

Is it ok to implement dict this way?