Handle multiple coins (at least 3 because of tricrypto)

[chanhosuh]

Factory pools are 2 coins but the code is mostly generic enough to handle more, e.g. tricrypto. A few places in the copied vyper factory contract highlights where the n-coin logic was changed to 2 coins. Those should likely be the only changes needed. For testing, we should use two vyper contracts based on n = 2 or 3: 1) existing fixture based on factory cryptopool 2) tricrypto2 fixture

Todo:

• create tricrypto2 fixture
• create test suite for n=3 checking against tricrypto fixture
• update CryptoPool to handle n=3

[chanhosuh]

tricrypto contract: https://github.com/curvefi/curve-crypto-contract/blob/master/contracts/tricrypto/CurveCryptoSwap.vy

[chanhosuh]

For now, we just need to test that swaps work properly to enable sims. The more complex logic in adding and removing liquidity can be checked later, as part of an eventual release for full pythonic interaction.