* `Address.toPayable`: added a helper to convert between address types without having to resort to low-level casting. ([#1773](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1773))
* Facilities to make metatransaction-enabled contracts through the Gas Station Network. ([#1844](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/1844))
* `Address.sendValue`: added a replacement to Solidity's `transfer`, removing the fixed gas stipend. ([#1961](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1961))
* `Address.sendValue`: added a replacement to Solidity's `transfer`, removing the fixed gas stipend. ([#1962](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1962))
### Improvements:
* `Address.isContract`: switched from `extcodesize` to `extcodehash` for less gas usage. ([#1802](https://github.com/OpenZeppelin/openzeppelin-solidity/pull/1802))