Drop bug fix: replace MerkleProofindex with type casted address of claimer

[nkrishang]

Before

// Setting the BitMap value when an allowlisted claimer claims tokens
claimCondition.limitMerkleProofClaim[activeConditionId].set(merkleProofIndex);

// Checking if 'BitMap value' has been set for a claimer. This 'BitMap value' is the `merkleProofIndex` of the claimer.
require(!claimCondition.limitMerkleProofClaim[_conditionId].get(merkleProofIndex), "proof claimed.");

After

address claimerAddress;

// Setting the BitMap value when an allowlisted claimer claims tokens
claimCondition.limitMerkleProofClaim[activeConditionId].set(uint256(uint160(claimerAddress)));

// Checking if 'BitMap value' has been set for a claimer. This 'BitMap value' is the type casted of the claimer.
require(!claimCondition.limitMerkleProofClaim[_conditionId].get(uint256(uint160(claimerAddress))), "proof claimed.");

Reason for the change.

In the After case, the BitMap value for a claimer x claim_condition is unique for the claimer. This is different from using merkleProofIndex, since that can be the same for 2 different addresses (each included in a separate allowlist of separate claim conditions, but at the same position i.e. with the same condition Id).

[joaquim-verges]

can we add a couple of tests for this? can be in the SDK too if its easier.