Make sdk.Coins.Add more efficient

[sunnya97]

As proposed by @ValarDragon in #2385

Currently our coins.Add method doesn't take advantage of the lists being sorted, so this takes O(N) time effectively...We should probably implement the logarithmic coins adding for this case then, or just make coin addition time less biasable in general.

[jackzampolin]

I think this has been addressed. Closing. cc @alexanderbez

[ValarDragon]

It is not. https://github.com/cosmos/cosmos-sdk/blob/master/types/coin.go#L223

[kochetkov-av]

It looks like current implementation fully relies on coins being sorted lexicographically by coin.Denom and couldn't be optimized significantly.

[ValarDragon]

Agreed, the only place where we could get benefits is from the case where one adds a set of coins with many entries, and one with few entries. However since its stored in a sorted array, insert must be O(N), so there doesn't seem to be a notable performance benefit to be had.