ArkProject: NFT Bridge

In `erc721_bridgeable.cairo::set_token_uri`, an unused variable _`exists` has been applied.

The failure to declare this variable at the beginning poses a serious threat to the contract. `_exists` is supposed return a boolean `true` or `false`. A malicious user can define the variable to their needs. After doing so, they can custom the metadata to their preference, thus ending up manipulating the protocol and stealing.

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The attacker could manipulate `set_token_uri`, set fake URIs and steal from other honest users.

Manual review

Declare `_exists` variable at the struct `Storage`