Modified summation symbols and big versions for operators like \ast or \square

[topological-modular-forms]

There are several $\sum$-like symbols commonly used but not natively supported by LaTeX, like:

1. Modified summation symbols $\sum'$, as requested by Timothy Chow.
2. Primitive summation symbols ${\sum}^*$, as in TeX SE 635008.
3. A \sum version of $\ast$, for e.g. iterated free products of rings, as requested by Yves de Cornulier.
4. A \sum version of $\square$, for e.g. iterated funny products of categories.
5. A \sum version of $\times$, as requested by Brauer Suzuki.