[CL][Swap Router Module]: implement store index to map pool id to swap module

[p0mvn]

Background

We need to store a mapping from pool id to swap module (gamm or concentrated-liquidity).

This is necessary for swaps. Given the pool id, we should be able to know which module to route the swap to.

Currently, swaprouter hardcodes all swaps to be routed to gamm.

Suggested Design

See for details: #3233

1. Implement:

// x/swaprouter/types/keys.go
var	SwapModuleRouterPrefix     = []byte{0x01}
type ModuleRoute struct {
    PoolId uint64
    PoolType PoolType
}
// ModuleRouteToBytes serializes moduleRoute to bytes.
func ModuleRouteToBytes(moduleRoute ModuleRoute) []byte {
    ...
}
// ModuleRouteFromBytes deserializes an encoded module route. It returns
// an error if decoding fails.
func ModuleRouteFromBytes(bz []byte) (ModuleRoute, error) {
    ...
}

2. Use the above to store the mapping in CreatePool

• update tests to check that the store is updated

3. Implement pseudocode.

// x/swaprouter/router.go RouteExactAmountIn(...)
moduleRouteBytes := osmoutils.MustGet(swaproutertypes.FormatModuleRouteIndex(poolId))
moduleRoute, _ := swaproutertypes.ModuleRouteFromBytes(moduleRouteBytes)
swapModule := k.routes[moduleRoute.PoolType]
_ := swapModule.SwapExactAmountIn(...)

4. Optionally in the same PR, complete: [CL][Swap Router Module]: test CreatePool #3236

5. Optionally in the same PR: [CL][Swap Router Module]: improve TestMultihopSwapExactAmountIn and TestMultihopSwapExactAmountOut #3131

Acceptance Criteria

• suggested design implemented
• proper abstractions added, not just pseudocode
• requested tests added

[p0mvn]

Before proceeding, consider alternatives from: #3097

[p0mvn]

fixed by #3097