* `Strings`: add `toString` method for signed integers. ([#3773](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3773))
* `Strings`: add `toString` method for signed integers. ([#3773](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3773))
* `MerkleProof`: optimize by using unchecked arithmetic. ([#3745](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3745))
* `MerkleProof`: optimize by using unchecked arithmetic. ([#3869](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3869))
* `EnumerableMap`: add a `keys()` function that returns an array containing all the keys. ([#3920](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3920))
* `EnumerableMap`: add a `keys()` function that returns an array containing all the keys. ([#3920](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3920))