Free purchasing not always desired

[cxkoda]

In many use-cases free of charge minting by the owner might not be desired. Using Seller.so would therefore introduce an unnecessary deployment overhead.

A possible solution would be to split off the free purchase functionality into an own contract - increasing composeability.
The number of sellable pieces will then be configured exclusively via the totalInventory setting. It is now in the responsibility of the consumer to account for possibly reserved pieces in the configurations.