Use reducible_balance instead free_balance

[tgmichel]

Rel #291

Follow up on Add a usable_balance_for function to the Currency trait.

Once we update frontier branch we should be able of using https://crates.parity.io/frame_support/traits/tokens/fungible/trait.Inspect.html#tymethod.reducible_balance, that returns the balance that is neither reserved nor locked/frozen.

@sorpaas in the EVM context, usable balance should take into account ExistentialDeposit? That is, should allow to go below it?

[JoshOrndorff]

in the EVM context, usable balance should take into account ExistentialDeposit? That is, should allow to go below it?

At first I thought having the reducible balance not include the ED would be a great way to have the benefits of a non-zero ED and still keep eth compatibility. But now I think that's wrong because of a bad token transfer experience.

Consider a chain with a ED = 1 token and the reducible balance does not include the ED. Imagine Alice has a total of 10 tokens, so from the EVM it looks like she has 9. Imagine Alice transfers 5 tokens to Bob. Because Bob also has a 1 token ED it appears that he only got 4 tokens.

EDIT: This would only be true if bob's account is new.

[sorpaas]

I'll do a final round of partial release for whatever crate is still possible. Then let's finally move Substrate to master and wait for paritytech/substrate#9140

[tgmichel]

@sorpaas just a reminder that this and #403 are stale waiting for Substrate's frontier branch :)

[sorpaas]

This is now merge-able. Please pull master.

[sorpaas]

Please fix tests and lints.