Rename `recursive_iterator` to `recursive_generator`

It's a much clearer name for describing what the function does. The old name would stick around but be deprecated and not work in `--strict` mode as we've done previously for `of`, `datamaker`, and `prepattern`.

Search the Coconut source for `recursive_iterator` and inspect how the existing deprecations for `of`, `datamaker`, and `prepattern` are implemented. Trace the strict-mode checks and related tests or documentation. Done means `recursive_generator` is supported, the old name remains deprecated, and strict mode rejects the old name consistently.

Written by the indexing model from the issue text.