Hashfield

So here is the first stable version.
The main controversial change is the refactor of the internal index API.
My main goals where:
- remove the ugly _index_and_traverse_command
- remove the _to_(de)index and _pre/_post_callback
- remove proxy_commands
- keep readable the specificities of all the commands
- more splitting concerns in subclasses to prepare the HashField

It's up to you know :)