Native balance refactoring

[maurolacy]

Refactor cw1-subkeys native Balance into cw0.

Use it in the cw20-atomic-swap contract Balance.

Closes the native part of #76.

[ethanfrey]

Looks quite good. I just left some comments on naming and some usage details to make this a bit clearer. Logic looks good. Except maybe the is_empty

Also, I would rebase on master since #86 was merged.

[maurolacy]

I like how it is now. Normalizing is a relatively costly (and mutable) operation, and in this way we don't enforce it on every is_empty() call.

Also, with your proposal for checking for empty balances using an iterator, is_empty is now independent of normalization.

Finally, I moved that is_empty() check to the NativeBalance struct directly.

[ethanfrey]

Looks good, I would just remove the normalize and merge

[maurolacy]

With the change to is_empty below, you can remove this line, right?

You can, in the sense that the is_empty() check will still work. That is, it doesn't depend on a previous normalization anymore.

But, keeping it does no harm, as normalization is a good thing, i. e. it's better to store a normalized balance that an un-normalized one.

I just didn't want to couple the normalization with the empty check, because of, you know, purity. :-)

Suppose in the future we notice that normalizing is an expensive operation, by example when you have a large list of coins. We can add a check / modification to the main contract code, to account for that. That is: now we can change normalization, and normalization usage in a per-contract basis, without affecting is_empty() at all.

[ethanfrey]

But, keeping it does no harm, as normalization is a good thing, i. e. it's better to store a normalized balance that an un-normalized one.

This is a good point and if we did other manipulations on the array (like with escrow), I would add it.

For now, I would remove it and save the gas costs. And we use that where we want to do more array manipulations (and maybe optimize all the NativeBalance ops to work with normalize balances)