Add rust-based unit tests to AMM

[metaproph3t]

The AMM swap and TWAP logic has been put in struct implementations, which means we can test it from Rust-based unit tests.

We have some unit tests, but they're extremely basic and don't really test any edge cases (e.g., large amounts, small amounts, large combined with small, et cetera). It would be good to expand these to make them more comprehensive.

[metaproph3t]

It looks like GitHub doesn't allow me to assign you directly, but consider yourself assigned @0xbigz!