ERC20Vesting: vesting functionality for ERC20 tokens

🧐 Motivation
Escrowed tokens are omnipresent in DeFi, as governance, airdrops and liquidity mining use vesting to control dumping. I suggest adding an ERC20Vesting extension that easily allows vesting tokens.

📝 Details

Vesting can be used in particular in combination with ERC20Wrapper.
The vesting schedule is linear by default but can be overriden (we use some exponential for our own vesting schedule).
The logic when withdrawing isn't fixed: the extension returns the amount already vested and the amount remaining, up to the implementation to choose whether to allow early unlock and which penalty to apply.

I think it would make sense to add some additional virtual function to handle the data field, or to modify a schedule (eg, to allow partial withdrawal).
The function names differ from VestingWallet, maybe it would make sense to harmonize those.
Please give me feedback!

Draft implementation: https://github.com/Keref/openzeppelin-contracts/blob/master/contracts/token/ERC20/extensions/ERC20Vesting.sol

Start by reviewing the linked draft ERC20Vesting implementation and comparing it with the existing VestingWallet and ERC20Wrapper APIs. Clarify the vesting schedule, withdrawal behavior, data handling, and function naming before proposing an implementation; done requires agreement on the extension's scope and API.