Add a stats/1 callback to all datatypes.

The callback returns a proplist of atom/number pairs about internal statistics of the datatype. Examples include 'actor_count' (on any VV-based type), 'element_count' (sets), 'field_count' (maps), 'max_dot_length' (on dotted VV-based types).

Still undecided: whether to recurse into embedded values for internal stats.