Exploit potential parallelism in the `InMemoryBackend` for `LargeSmtForest`

### What should be done?

The `InMemoryBackend` is designed to be usable in contexts where parallelism is impossible. However, where it is, it would be nice to exploit said parallelism.

### How should it be done?

Examine the use of `maybe_rayon` to exploit parallelism where it is available.

### When is this task done?

When possible points for parallelism can take advantage of it when compiled with support.

### Additional context

_No response_

Look at the InMemoryBackend and LargeSmtForest code to understand the data structures. Identify loops or operations that could be parallelized using maybe_rayon. Check how parallelism is conditionally compiled in the project. Done when the backend uses parallelism where available without breaking single-threaded contexts.

Written by the indexing model from the issue text.