Support for infinite terms

Rewrite should be able to store and normalize [infinite terms](https://en.wikipedia.org/wiki/Infinite_expression). The following forms seem especially important, although the implementation of infinite terms overall should ideally be generalized to fit these and more.

- [ ] Infinite sums and products
- [ ] Infinite nested radicals
- [ ] Infinite power towers
- [ ] Infinite continued fraction

```julia
julia> t = Sum(@term(k), @term((-1)^k / factorial(2k + 1) * x^(2k + 1)), 0, Inf);

julia> normalize(@term(diff($t, x)))
Sum(@term(k), @term((-1)^k / factorial(2k) * x^(2k)), 0, Inf)
```

No file, test, or entry point is named. Start with the Julia example and the linked definition of infinite expressions, then locate the repository’s term and normalization code. Done should cover the listed infinite sums and products, nested radicals, power towers, and continued fractions, including the shown derivative normalization.

Written by the indexing model from the issue text.