Support Curve stableswap pools

[FlattestWhite]

Code Review Processes

Curve pools can have 2, 3 or even 4 coins in a pool. However, from an exchange perspective, we'd only ever be swapping two coins at one time. Furthermore, most coins in all pools are ERC20 tokens. Hence the eth/steth pool needed some extra logic around wrapping and unwrapping for eth/weth. The approach taken with this adapter is to assume we're only going to be trading ERC20 tokens.

It assumes both tokens that are being traded are ERC20 tokens and it requires the index on of the coin on the curve pool to be passed in at construction. The rest is just a simple encoding of the exchange function.

Newer curve pools also have an exchange function that can take a destination address as the last argument. This means we can save an extra contract call. However, most existing curve pools don't support that new exchange method. Hence, I've decided to use the older more widesperad "exchange" method, but it requires doing an extra transfer of tokens back to the user.

New Feature Review

Before submitting a pull request for new review, make sure the following is done:

• Design doc is created and posted here: [Insert Link]
• Code cleanliness and completeness is addressed via guidelines

README Checks

• [] README has proper context for the reviewer to understand what the code includes, any important design considerations, and areas to pay more attention to

Code Checks

• [] Add explanatory comments. If there is complex code that requires specific context or understanding, note that in a comment
• [] Remove unncessary comments. Any comments that do not add additional context, information, etc. should be removed
• [] Add javadocs.
• [] Scrub through the code for inconsistencies (e.g. removing extra spaces)
• [] Ensure there are not any .onlys in spec files

Broader Considerations

• [] Ensure variable, function and event naming is clear, consistent, and reflective for the scope of the code.
• [] Consider if certain pieces of logic should be placed in a different library, module

[FlattestWhite]

Added forked integration tests - and fixed up the assertions so they're passing. Also added comments to explain why the assertions are the way they are.

[FlattestWhite]

LGTM! Thanks for the additional tests and the clarifying comments - very helpful.

Thanks - will your team merge and write the deploy script? Or will you need me to do so?

[cgewecke]

@FlattestWhite Just merged - we will deploy the adapter. I've asked some questions about how to parameterize the deployment in a slack channel Set shares with IC which Ed added you to yesterday.

Will continue discussion there :)

[cgewecke]

:tada: This PR is included in version 0.10.3-hhat.1 :tada:

The release is available on:

• npm package (@hardhat dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: