Math improvements

[julienrbrt]

This is a tracking issue for math improvements (for a potential math v2):
Some of those changes cannot be integrated in math v1 due to their possible state machine breaking nature.

• MarshalJSON(LegacyDec{}) != MarshalJSON(LegacyNewDec(0)), which leads to some issues:
  • [Bug]: Amino signature verification fails if message contains empty sdk.Dec #17606: fix(math)!: make LegacyDec zero values marshaled equal #17613
  • [Bug]: math.LegacyDec nil values cannot be symmetrically marshaled and unmarshaled #21808
• upstream GDA based decimal type #11783
• Improve API by avoiding mutation methods: [Feature]: seperate mutative from immutable api for Int #18066