Unexpected "negative coin amount" in DecCoin's Sub, Suggest sort before sub.

[readygo586]

Summary of Bug

Upexpected negative coin amount in DecCoin's Sub

When call DecCoins's Sub, if not sort the coins before, an unexpected "negative coin amount" happen.

Version

Cosmos-sdk v0.37.2

Steps to Reproduce

Run the following test code, a panic happens,.

func TestSubDecCoins(t *testing.T){
	decCoins1 := NewDecCoins(Coins{NewCoin("mytoken", NewInt(10)),  NewCoin("btc", NewInt(20)), NewCoin("eth", NewInt(30))})
	decCoins2 := NewDecCoins(Coins{NewCoin("btc", NewInt(10)),  NewCoin("eth", NewInt(15)), NewCoin("mytoken", NewInt(5))})
	diff := decCoins1.Sub(decCoins2)
	require.Equal(t, decCoins2, diff)
}

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned

[alexanderbez]

Thanks @readygo586!

Unfortunately, our decimal coins implementation didn't receive as much TLC as our coins implementation did. All arithmetic assumes the invariant that the coins are valid and sorted. We need to update documentation and update NewDecCoins.