Modern C++ Syntax Cleanup

Since we're using C++23, let's upgrade our for loop iterations for readability. (These changes don't impact performance.)
* Use `iota` (C++20) for range-based for-loops that are purely index based
* Use `enumerate` (C++23) for range based for-loops that are both index an object based.
* Use `span` (C++20) for passing around range-based iteration.