Decide on high level looping constructs

So far what I'd like to implement is:

_Functional Looping_
- `Map`
- `Filter`
- `Reduce`

_Mathematical Looping_
- `Sum` (not like sympy's sum, more like python's sum)
- `Prod` (like `Sum`, but with `*`, not `+`)

Ideally, sympy's `Sum` could also work, so maybe we should have `sum` instead, that returns a `sympy.Sum` object, with the iteration setup properly.

Start with issue #3, which proposes functional constructs Map, Filter, and Reduce plus mathematical Sum and Prod. Resolve the open API question around a Python-like sum versus a SymPy Sum before locating implementation entry points. Done means the looping construct design is agreed and the listed operations have a defined, consistent interface; no files or tests are named in the issue.

Written by the indexing model from the issue text.