[`experimental` feature]: Potential data race when using `CompressionPolicy::High` with `Freelist::Optimistic` or `Freelist::Pessimistic`

[al8n]

cargo miri reports data race when using CompressionPolicy::High with Freelist::Optimistic or Freelist::Pessimistic and a reclaim happens in the CAS error branch, not sure if the data race happens because of this crate or the rarena_allocator crate.

Only allow users to customize CompressionPolicy and Freelist when the experimental feature of this crate is enabled for a temporary solution.

[al8n]

Seems that the data race was caused only by the CompressionPolicy::High is enabled, see #38.