Check for / reject zero amount tokens

[maurolacy]

Add a check for considering zero amounts as missing.

For native coins, filter out zeros, and then ensure then remaining balance has length > 0. Only save the filtered list.

For cw20 coins, reject zero amounts directly. Add a check to also reject such sends in cw20-base.

[ethanfrey]

As part of this, let's pull out the Balance helper from cw1-subkeys into cw0 (as it only relies on cosmwasm-std and used by many different protocols).

This already has a normalize function which you could use (or add a more efficient is_empty one).

This would be a great help to build the higher-level checks for native coins in all contracts.

I would also suggest 2 PRs here: one for the cw20 additions, one for the native coin changes (that I mentioned here)

[maurolacy]

Yes, I was taking a look at the subkeys code, and was going to suggest exactly that.

[ethanfrey]

This is done with #86 and #87, correct?

[maurolacy]

Correct.