Finally, move the subtype predicate storage to the Variable.

Since the predicate subtype can be set per variable, move that logic to the
Variable class, away from the Type class.  Before, Type disagreed on whether
two instances could be equal, and whether they could be easily duplicated.
This makes the distinction unimportant.  It does neuter the custom type
functionality in VariableParser some, but that can be added back later.