prevent duplicate denoms

[ivmidable]

This PR prevents token1 and token2 from being the same Denom and also refactors create_amm in integration tests to take in a Denom to make it easier to test for this behavior. two new tests were also added to make sure this fix works as expected. A AMM that allowed token1 and token2 to be the same token lost users around 31 million USD in the past.

https://slowmist.medium.com/detailed-analysis-of-the-31-million-monox-protocol-hack-574d8c44a9c8

[0xekez]

hey! thanks for the PR. i've also seen that medium article / exploit.

these contracts are not vulnerable to that problem afik. creating an AMM where both pools have the same denom will actually work as expected: you put denom in, you get denom - fees out.

i'd love to land a test that tests that case, but i'm not sure it's worth adding the additional validation to the contracts. in smart contract land, more code == more bugs. if the state that we're protecting against doesn't do anything unexpected i figure the best thing to do is to allow it. :)

will defer to @ben2x4 about if we'd like to land this whole thing (i think it looks good), but, again, if you'd like to just break this out into a test that tests the multiple denom case that would be a great contribution!