Change Coin storage model

[robert-zaremba]

Summary

The sdk.Coin has the following type definition:

type Coin struct {
	Denom  string 
	Amount Int
}

Bank module stores account balances in KV store using following schema:

key = "<module key>/<holder_address>/<denom>"
value = proto.Marshal(coin)

Problem Definition

The denom is repeated both in the key and value and wastes the valuable blockchain space.

Proposal

We can save a blockchain space if we only save the amount in value:

key = "<module key>/<holder_address>/<denom>"
value = coin.Amount.Bytes()

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[alexanderbez]

I think the proposal is fine 👍

[aaronc]

That makes sense, but why do we even need to proto.Marshal a string? Why don't we just do store.Set(key, []byte(coin.Amount))

[robert-zaremba]

yes, we can save amount directly without using protobuf.