Add AddAmount/SubAmount method to sdk.Coin

[hallazzang]

Summary

sdk.Coin has Add() and Sub() methods but I think it'd be good to have AddAmount() and SubAmount() methods too.

Problem Definition

If someone wants to subtract some amount from existing coin, one should right:

amount := sdk.NewInt(1000)
result := existingCoin.Sub(sdk.NewCoin(existingCoin.Denom, amount))

But that's a bit redundant that we have to type existingCoin twice.
We should do it better with:

amount := sdk.NewInt(1000)
result := existingCoin.SubAmount(amount)

And it'll improve code readability a lot.

Proposal

Implement AddAmount() and SubAmount() to sdk.Coin.

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For Admin Use

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[amaury1093]

This sounds okay to me, would you like to create a PR?

[hallazzang]

@AmauryM Yep, I'll create PR right away.