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.
Internal storage capacity of the dictionary is initialized using the
Default_size value. Then, tuning of needed storage capacity is performed automatically
according to usage. If you are really sure that your dictionary is always really bigger than
Default_size, you may consider to use with_capacity to save some execution time.