Renaming __metadata__ to metadata

In the call we discussed renaming __metadata__ to metadata and we decided to go with this change.

The worry expressed was that __metadata__ might be in the future used by Python itself and we might have issues because of that. The downside is that it might happen that some value we want to wrap might use metadata for its purpose. It seems that current values we are considering do not do so.

As a consequence, we will make some attributes on primitives public, and declare that public attributes (those without _ prefix) should not be used to assure future compatibility.