Add toAssoc() and toList()

The `toArray()` method outputs an associative array. If you want the values, you explicitly need to use the `values()` step. Adding specific methods for associative and sequential arrays would be better.

`toArray()` would be deprecated, or add parameter `?bool $preserveKeys = null` and pass it to `iterable_to_array()`.

Start by locating the existing `toArray()` implementation and the `iterable_to_array()` entry point. Review how associative and sequential results are currently represented, then clarify whether the intended API is `toAssoc()`/`toList()`, a `toArray()` parameter, or deprecation; the chosen behavior should be implemented and covered by the project's tests.

Written by the indexing model from the issue text.