[Feature]: seperate mutative from immutable api for Int

[tac0turtle]

Summary

There has been a need for more mutable apis in math, they have been getting added to the current struct which could cause confusion for users.

Problem Definition

separate immutable from mutable in math

Proposed Feature

Create a new mutable struct for users to use when needed.

New api to add:

• https://github.com/cosmos/cosmos-sdk/pull/18030
• https://github.com/cosmos/cosmos-sdk/pull/17803 (unreleased, can be reverted with the new implementation)

[julienrbrt]

Additionally, we can revert this (https://github.com/cosmos/cosmos-sdk/pull/17803, that hasn't been released and which would fit here. I'll edit your message above.

[testinginprod]

The proposal would be to introduce a new type: type IntMut Int  which can be converted from and to Int with little to no overhead by doing (IntMut)(Int) and vice-versa (Int)(IntMut), then the mutative API would be implemented on top of IntMut, this would reduce the API surface of Int which is already big and support the mutative API in a performant way.

Eg:

type IntMut Int

func (i IntMut) ToInt() Int { return (Int)(i) }

func (i IntMut) Quo/Add/Sub/Etc() { ... } 

func (i Int) ToIntMutUnsafe() IntMut { return (IntMut)(i) }
func (i Int) ToIntMut() IntMut { return NewIntMutFromBigInt(i) } // this NewIntMutFromBigInt would copy the underlying Int.bigInt

[julienrbrt]

FWIW comment from the bot: https://github.com/cosmos/cosmos-sdk/pull/18247#discussion_r1371130502