Implement AD in interpreter

[athas]

The Futhark interpreter is an important development tool, but it does not currently support AD. Since our AD transformation is based on transforming the core language, it cannot be used in the interpreter, which works by design and intent on an un-molested source language representation. We could implement AD in the interpreter by simply constructing the Wengert lists in the naive and "obviously correct" manner. This would not be particularly efficient (in particularly in terms of space), but it could probably be done in a non-invasive manner, such that later changes to the interpreter don't all have to worry about how it fits with AD. It would also serve as a great tool for testing the correctness of the more optimised AD compiler transform.

[athas]

Implemented in #2186.