Gamm stableswap improvements

[ljah8]

What is the purpose of the change

This PR is created during the Informal Systems audit, after analysis of the existing specification and code inspection
Auditing is performed on commit hash: 7374795
This PR contains minor Stableswap code improvements within the GAMM module noticed during code inspection with the aim of finding issues.

Brief Changelog

• stableswap/pool.go:
  • Part of the code used to build errors is moved to errors.go;
  • The getLiquidityIndexMap and GetScalingFactorByLiquidityIndex functions are replaced by the GetScalingFactorByDenom function to get the appropriate scaling factor faster and avoid creating a map. While implementing the new function, we didn't want to change logic, so it doesn't return an error or panic if no scaling factor is found.
  • applyScalingFactorMultiplier function is protected against unwanted overflow in case multiplier > 1
• Minor changes in the implementation of BinarySearchBigDec, BinarySearch (osmoutils/binary_search.go) and joinPoolSharesInternal (stableswap/amm.go) functions.
• Removed redundant error from iterKCalculator function that always returned nil.
• Added checking for potential division by zero in Compare and CompareBigDec functions.

Testing and Verifying

• Added a unit test covering the GetScalingFactorByDenom function.
• Existing tests are modified in accordance with the new changes.

Documentation and Release Note

• Does this pull request introduce a new feature or user-facing behavior changes? no
• Is a relevant changelog entry added to the Unreleased section in CHANGELOG.md? no
• How is the feature or change documented? not applicable

[AlpinYukseloglu]

LGTM, thanks for the cleanup!

[ValarDragon]

LGTM! I had previously suggested backporting this.

Even though this looks like it should be fine, changing to no backport just to be abundandantly cautious with the division by zero change. (I'm pretty sure it should be impossible to get a zero in current code)