randomly assign box art on nft mint

on mint randomly assign box art to the newly minted token

there will be 100 statically generated types of box art available on ipfs.

there should not be more than 100 instances of any individual pice of box art

implementation details

as part of #12 it is expected that we will receive 100 IPFS addresses for storage in the contract
the 100 ipfs addresses should be stored in an index based array
when a box is minted, it should randomly select an integer [0,100) which corresponds to an index in the array.
the random integer should be stored in a mapping with the token
when tokenUri is queried, the index associated with the token should be queried and then the ipfs address should be returned from the array
randomization
the randomization can be something simple like a modulo9 operation on the block and timestamp
we need to maintain a mapping of each index in the array and the number of instances that have been generated
there should never be more than 100 boxes of any type
on random number generation, if there are already 100 boxes of that type, we need to pick another box instead (implementation details are unclear)