Introduce a scalar version of `Scan`

[brandonwillard]

As @ricardoV94 mentioned in #512 (comment), a scalar-compatible version of Scan would allow us to use Scans within Elemwise.scalar_op implementations and help us close #512.

We should first determine what it would take to refactor Scan so that it can work exclusively with ScalarType inputs and outputs before considering alternative approaches (e.g. a new Op).