[x/gamm][stableswap]: Increase BigDec precision even further to accommodate large 4-asset pools

[AlpinYukseloglu]

Background

Even with 36 decimal places of precision, large 4-asset pools still overflow. We should increase precision to whatever number allows us to comfortably support $100B per asset in a 4-asset pool.

Suggested Design

• 72 precision should be enough, but we might need to increase it even further

Acceptance Criteria

• All new and existing tests pass

[AlpinYukseloglu]

Update: now that we calculate our multi-asset CFMM using k' = k / u, this is no longer an issue and 36 precision allows us plenty capacity