[stableswap]: Convert all core stableswap arithmetic to use BigDec

[AlpinYukseloglu]

Closes: #2776

What is the purpose of the change

This PR updates all core arithmetic operations in stableswap to use our BigDec type instead of the regular sdk.Dec type.

Brief Changelog

• Update all uses of sdk.Dec for internal calculations to use BigDec
• Add methods for converting from sdk.Dec to BigDec and vice versa to our BigDec implementation

Testing and Verifying

• The core change is currently a trivial refactor as sdk.Dec and BigDec have the same precision, but this change will require more thorough testing once we increase precision bounds in BigDec
• The conversion methods for sdk.Dec <-> BigDec only have basic sanity tests, as testing them more thoroughly requires finalizing the specific decimal precision we want for BigDec (left for a future PR to not block this one)

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? (yes / no)
• How is the feature or change documented? (not documented)