Radix tree implementation

Radis is a little library to provide an implementation of a Radix tree (specialized with a scalar key - like a string, bigarray or array). This project is a part of ocaml-git to have a fast access to an immutable store. In this way, remove operation should be the slowest operation and this data-structure is focused on lookup operation when the key is specifically a hash (see digestif).

The idea behind this library is to provide a fast access to an immutable store (like git). So, in my mind, if we put a new object in this store, it can not be deleted. It's why remove operation should be slow - of course, in a git store, it's possible to delete an useless object (see git gc), however this computation does not appear in this way.