Add ERC721 support

[andreivladbrg]

Closes #47 #73 #66 #63

Changes

• inherit ERC721 in SablierV2OpenEndedState
• implement _update hook
• implement _isCallerStreamRecipientOrApproved helper function
• inherit IERC4906 and add updateMetadata modifier
• introduce a new amount (remainingAmount) in Stream struct
• remove balance subtraction in _extractFromStream function
• add isTransferable bool
• mint the NFT in _create function
• remove ERC20 transfers from cancel and adjustRatePerSecond function
• allow withdraw function to be called after the stream is canceled
• update withdrawableAmountOf function to include remainingAmount
• use modifiers only in external/public function
• add Access Control table in README
• update invariants in README
• update tests accordingly to the above changes in src

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Also, I've created some useful diagrams, but they will be correct once we make the cancel --> pause change:

Diagrams

State diagram

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Calculation diagrams

[smol-ninja]

Since its a huge PR and there are so many ongoing changes for other open issues, should we merge it after the following review and start working on top of it? We can create more smaller issues if we find problem with it later. cc @andreivladbrg wdyt?

PS: I have't reviewed the tests yet.

[smol-ninja]

While reviewing this PR, I have realized that it would have been easier to review it if it had been split into multiple PRs.

• 1 PR introduces ERC721 implementation
• 1 PR introduces withdrawableAmountOf changes
• 1 PR introduces isTransferable

What do you think @PaulRBerg @andreivladbrg? imo we should avoid large PRs until its absolutely necessary.

[PaulRBerg]

agree, let's split in multiple PRs

[andreivladbrg]

@smol-ninja I've pushed a new commit addressing some of your feedback

And, as we discussed privately, we will merge this PR as is (to make things moving faster), and we will create future issues in case of problem

[smol-ninja]

yeah sounds good.