[x/gamm][stableswap]: Consider making CalcJoinPoolShares not mutate pool assets by factoring out liquidity updates

[AlpinYukseloglu]

Background

Our balancer implementation has separate functions for calculating pool join shares (CalcJoinPoolShares) and actually updating pool assets/liquidity (IncreaseLiquidity), whereas our stableswap implementation does all of it in CalcJoinPoolShares -> joinPoolSharesInternal.

These two implementations are inconsistent, so it would make sense to change one of them to fit the other. It's not clear to me which implementation is cleaner, but I am leaning towards factoring out the pool state updates from our stableswap implementation into a separate function like in balancer.

Note: our balancer implementation seems cleaner at a first glance, but it also requires setting up a local pool asset map to keep track of intermediate changes which adds a nontrivial amount of complexity to an already large function.

Suggested Design

• Create a local pool asset map in joinPoolSharesInternal and redirect intermediate liquidity updates to local updates of this map
• Remove any other liquidity updates at the end of joinPoolSharesInternal
• Create a separate IncreaseLiquidity function that updates pool liquidity and add it where relevant (e.g. after CalcJoinPoolShares in JoinPool)

For reference, the main two places where pool assets are mutated are here and here

Acceptance Criteria

• joinPoolSharesInternal (and therefore CalcJoinPoolShares) does not mutate pool assets
• Existing tests pass

[AlpinYukseloglu]

Turns out this version is actually cleaner than what we have in Balancer & we should probably consider changing that implementation instead!