Actually, this feature is useful only
when the type of values (the type V_) is a reference type, to avoid using has just
followed by at to get the corresponding value with the very best performances.
If there is as yet no key k in the dictionary,
enter it with item v. Otherwise overwrite the item associated with key k.
As the put procedure actually uses is_equal, you may consider to use fast_put for expanded
objects as well while trying to get the very best performances.
Remove entry k (which may exist or not before this call).
As the remove procedure actually uses is_equal, you may consider to use fast_remove for expanded
objects as well while trying to get the very best performances.