preliminary scalarset support

We now recognise scalarsets as a separate type from ranges. Code generation for
them still works the same as ranges; that is, there's no support for actual
symmetry reduction yet.

This commit fixes a misunderstanding where scalarsets were actually mapped to a
larger range than they should have been previously. That is, the scalarset bound
was considered an upper bound rather than one beyond the upper bound.

Github: Related to #12 "symmetry reduction"