Add an ERC721Enumerable extension to the contract

[PopcornPaws]

Description

The frontend needs a tokenOfOwnerByIndex method from the contract which returns the pin_id of a given user, based on the index of the pin among the user's tokens. To understand better, here's an example flow of how the respective storage map should look like:

• the user mints three pins with ids 234, 240 and 345

address -> balance_id -> pin_id
0xabc.. -> 0 -> 234
0xabc.. -> 1 -> 240
0xabc.. -> 2 -> 345

• the user burns pin 234

address -> balance_id -> pin_id
0xabc.. -> 0 -> 345 // the last pin id is written to the burned balance id
0xabc.. -> 1 -> 240

• the user mints pin 567

address -> balance_id -> pin_id
0xabc.. -> 0 -> 345
0xabc.. -> 1 -> 240
0xabc.. -> 2 -> 567

@TomiOhl does this look correct?

[TomiOhl]

Looks good! Let me add some reference links:

• the enumeration extension referenced in EIP-721
• OpenZeppelin's implementation in Solidity