[x/gamm][stableswap]: Simulator/fuzz testing for stableswap

[AlpinYukseloglu]

Background

We should make sure that we have fuzz testing for our core invariant and ideally for our binary search implementations (mainly single asset join) to ensure they function across a full range of inputs

Suggested Design

• Invariant testing (ensure that k changes in the right direction and by small amounts related to BigDec truncations or rounding error) – swap fees should be taken out of input assets so I believe they should not affect k value changes, so unless I'm missing something the main focus of this should be on ensuring rounding is handled properly
• Single asset join binary search (should be fine as LP share -> single asset out function is monotonically increasing, but worth testing anyway)

Acceptance Criteria

• All valid tests pass above reasonable thresholds