Remove dependency between cw20_escrow and cw20_atomic_swap

[ethanfrey]

There is an import in the escrow contract: use cw20_atomic_swap::balance::Balance;

We should be able to use cw0::Balance

Also, see if there is anything else to de-dup.

[maurolacy]

Just to be clear, for that I would have to move Balance (which is an enum for native and cw20 balances) to cw0. That would introduce a dependency of cw0 on cw20.

I'm OK with that, but want to confirm first. Maybe it's better to introduce another mod / namespace for these "mixed" structs.

Update: I would need to put Balance in cw20, to avoid circular dependencies.

[maurolacy]

#118. I think it's not bad to put there, as cw20 is built on top of cw0.

cw20 also lacked a balance mod so, we're all happy now.

[ethanfrey]

You are right... the native coin Balance was already in cw0, but this was a different one.

Great job placing it in cw20 in spite of my comment. Very clean