feature/accounts

[wangjj9219]

Closes #58

[wangjj9219]

ChargeTransactionPayment in pallet_transaction_payment doesn't expose the compute_fee ，So I copy the code.

https://github.com/paritytech/substrate/blob/7248ac20c4386b30804dcad1bdeeed4c0a71b6ca/frame/transaction-payment/src/lib.rs#L154-L155

Acala/modules/accounts/src/lib.rs

Lines 26 to 34 in ac3e5a4

	pub trait Trait: system::Trait + pallet_transaction_payment::Trait + orml_currencies::Trait {
	type FreeTransferCount: Get<u8>;
	type FreeTransferPeriod: Get<MomentOf<Self>>;
	type Time: Time;
	type Currency: MultiCurrency<Self::AccountId> + Send + Sync;
	type Call: Parameter
	+ Dispatchable<Origin = <Self as system::Trait>::Origin>
	+ IsSubType<orml_currencies::Module<Self>, Self>;
	}

And define type Call satisfied IsSubType<orml_currencies::Module<Self>, Self> ，If do not declare Trait derives from orml_currencies::Trait , will get following error ， is that ok？

error[E0277]: the trait bound `Self: orml_currencies::Trait` is not satisfied
  --> modules/accounts/src/lib.rs:26:1
   |
26 | / pub trait Trait: system::Trait + pallet_transaction_payment::Trait {
27 | |     type FreeTransferCount: Get<u8>;
28 | |     type FreeTransferPeriod: Get<MomentOf<Self>>;
29 | |     type Time: Time;
...  |
33 | |         + IsSubType<orml_currencies::Module<Self>, Self>;
34 | | }
   | |_^ the trait `orml_currencies::Trait` is not implemented for `Self`
   |
   = note: required because of the requirements on the impl of `sp_api_hidden_includes_decl_storage::hidden_include::Callable<Self>` for `orml_currencies::Module<Self>`
   = note: required by `sp_api_hidden_includes_decl_storage::hidden_include::IsSubType`

[xlc]

Looks reasonable. I will open a PR upstream to make compute_fee public

[xlc]

#72 should include paritytech/substrate#4669