[x/gamm][stableswap]: Make scaling/descaling logic also convert to and from BigDec

[AlpinYukseloglu]

Background

Scaling/descaling can serve as a convenient place to convert inputs and outputs to/from BigDec for core stableswap math. Once we make this change, we should ensure that the only conversions to and from BigDec happen through the scaling functions.

Suggested Design

• Make scaling functions do SDKDec -> BigDec and descaling do BigDec -> SDKDec

Acceptance Criteria

• All conversions to and from BigDec happen under scaling functions
• All existing and new tests pass

[AlpinYukseloglu]

It just occurred to me that this might not be fully possible since we directly interact with the PoolI interface for single asset joins @ValarDragon

Am I right in assuming that if we want to use scaling/descaling functions to gate SDKDec <-> BigDec conversions (and do all internal calculations with BigDec), we would have to create a BigDecCoin implementation and an internal pool interface that supports it?