Contiguous arena add remove test

[rezural]

This adds a test to ContiguousArena to add & remove. This test fails on master.

I'm not sure how to safely fix this, as I'm not too sure what this struct is doing, but the following fixes the panic:

        if let Some(rev_id) = swapped_rev_id {
            if let Some(indice) = self.indices.get_mut(rev_id) {
                *indice = i;
            }
        }