spender term in ERC20WithPermit is incorrectly used

[cygnusv]

Current implementation of ERC20WithPermit uses the term spender to refer to the source account from which tokens are transferred from, while both the EIP (see https://eips.ethereum.org/EIPS/eip-20) and major implementations (like e.g. OpenZeppelin, https://github.com/OpenZeppelin/openzeppelin-contracts/blob/a55b7d13722e7ce850b626da2313f3e66ca1d101/contracts/token/ERC20/ERC20.sol#L163) use the same term to refer to an account that's been authorized to spend tokens on behalf of another. This can lead to confusion (it almost happened to me recently preparing a DAO proposal), and has an ABI-compatible fix.

[mhluongo]

@cygnusv if you have a minute I'd love a spot check!

#18