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.
Approximation of the actual internal storage capacity.
The capacity will grow automatically
when needed (i.e. capacity is not a limit for the number of values stored). Also note that
the capacity value may not be always accurate depending of the implementation (anyway, this
capacity value is at least equals to count).