Data race and memory safety issue in `Index`

The appendix crate implements a key-value mapping data structure called
Index<K, V> that is stored on disk. The crate allows for any type to inhabit
the generic K and V type parameters and implements Send and Sync for them
unconditionally.

Using a type that is not marked as Send or Sync with Index can allow it
to be used across multiple threads leading to data races. Additionally using
reference types for the keys or values will lead to the segmentation faults
in the crate’s code.