Unsafe storage key in near_contract_standards

[hanakannzashi]

Storage key for non-fungible token is unsafe, this may be conflict with user's custom storage key if user define the same first storage key variant.
e.g.

#[derive(BorshSerialize, BorshStorageKey)]
enum StorageKey {
    MyKey {
        account_hash: Vec<u8>
    },
    // other variants
}

https://github.com/near/near-sdk-rs/blob/master/near-contract-standards/src/non_fungible_token/core/core_impl.rs#L54:L58