Add a VestingWallet

[Amxx]

Fixes #1214

Add a vesting wallet with a default linear schedule. The vesting schedule can be customized by overriding a single function.

PR Checklist

• Tests
• Documentation
• Changelog entry

[frangio]

We should check if the contract we're proposing supports the use cases listed in the comments in #1214.

[Amxx]

We should check if the contract we're proposing supports the use cases listed in the comments in #1214.

• "Graded vesting" can be implemented by overriding the release function with any shape/curve
• Monthly” & non-linear basis can also be implemented by overriding the release function. I have code for that, which is not part of the "small" PR but that we used to test.

[frangio]

We've decided to offer a single VestingWallet for both ERC20 and native token. The release function should operate the native token if the token address is a specific value to represent it, possibly 0x00...00 or 0xee...ee. Personally I'm leaning towards 0x0 because the "e" doesn't represent native token in other networks, and because the zero address is easily available as a constant in Ethers.js. Additionally there should be a separate release function for the native token, ideally with a different name so it doesn't cause function overloading.

[NeoXtreem]

@frangio What's the planned release date for this VestingWallet?