Improvements to List and Set in ds package

[mokiat]

The following changes are made:

• Adds a new ds.ListFromSlice for easier initialization of a *ds.List type.
• Adds Equals methods to *ds.Set and *ds.List types.
• Adds Unbox methods to *ds.Set and *ds.List types. This can be used for fast access to their inner representations. Potentially unsafe if misused.
• Adds a Set method to *ds.List so that an item at a specific index can be modified, similarly to how it can be retrieved with Get.
• Changes the behavior of *ds.List.Items method. Previously it returned the inner slice and now it returns a copy. Instead Unbox should be used.

WARNING The last change is a backward incompatible change. The impact is that existing apps will allocate memory where Items has been used until they migrate to Unbox. Furthermore, if existing apps did modify the slice (which hopefully they didn't as the documentation advised) as a means to mutate the List, that will no longer work correctly.

[Bo0mer]

LGTM