A lot of the work that was done by special-casing number types can now be
done by TR's handling of unions. This makes it easier to add more numeric
types to the tower and should make writing down types for numeric
primitives much less error-prone.
In addition, this commit adds several numeric types that will help get
tighter bounds on integer arithmetic, such as Index types. They will have
to be integrated to the base environment before they can be useful.
d4c93cc12e