Add a usable_balance_for function to the Currency trait

[sorpaas]

This adds a usable_balance_for function to the Currency trait. In situations like EVM, we care about the actually transferrable balance, which currently can't be fetched via the Currency trait. With this function we can do usable_balance_for(account_id, WithdrawReasons::TRANSFER).

[sorpaas]

^ It shouldn't need any Polkadot PR. The CI error looks like something else unrelated.

[gavofyork]

under what circumstance is this needed?

[sorpaas]

@gavofyork pallet-evm needs something like this. Right now, the EVM would return the account balance as free_balance. This, however, breaks the usual EVM assumption that if a balance is available, then it can be used in TRANSFER. By changing the returned balance to used_balance_for(TRANSFER) this issue can be then fixed.

[JoshOrndorff]

Is there any intention to merge this? If so the Moonbeam team is willing to brush the dust off of it. If this won't be merged we can begin looking for other solutions to polkadot-evm/frontier#291

[gui1117]

now that https://crates.parity.io/frame_support/traits/tokens/fungible/trait.Inspect.html#tymethod.reducible_balance exists I think this is no longer required no ?

Also this doesn't take into account the providers restriction. so people could expect to be able to withdraw the total amount where it is actually not possible.
Related paritytech/polkadot-sdk#335

[sorpaas]

@thiolliere Yeah I think you're right. This can be closed now!