Enumeratable accounts (#195)

* Merge remote-tracking branch 'origin/master' into gav-xts-dont-panic

* Update wasm.

* consensus, session and staking all panic-safe.

* Democracy doesn't panic in apply.

* Fix tests.

* Extra helper macro, council depanicked.

* Fix one test.

* Fix up all council tests. No panics!

* Council voting depanicked.

* Dispatch returns result.

* session & staking tests updated

* Fix democracy tests.

* Fix council tests.

* Fix up polkadot parachains in runtime

* Fix borked merge

* More Slicable support

Support general `Option` and array types.

* Basic storage types.

* Existential deposit for contract creation

* Basic implemnetation along with removals

* Fix tests.

* externalities builder fix.

* Tests.

* Fix up the runtime.

* Fix tests.

* Add generic `Address` type.

* Initial function integration of Address into Extrinsic.

* Fix build

* All tests compile.

* Fix (some) tests.

* Fix signing.

* Push error.

* transfer can accept Address

* Make Address generic over AccountIndex

* Fix test

* Make Council use Address for dispatch.

* Fix build

* Bend over backwards to support braindead derive.

* Repot some files.

* Fix tests.

* Fix grumbles

* Remove Default bound

* Fix build for new nightly.

* Make `apply_extrinsic` never panic, return useful Result.

* More merge hell

* Doesn't build, but might do soon

* Serde woes

* get substrate-runtime-staking compiling

* Polkadot builds again!

* Fix all build.

* Fix tests & binaries.

* Reserve some extra initial byte values of address for future format changes

* Make semantic of `ReservedBalance` clear.

* Fix panic handler.

* Integrate other balance transformations into the new model

Fix up staking tests.

* Fix runtime tests.

* Fix panic build.

* Tests for demonstrating interaction between balance types.

* Repot some runtime code

* Fix checkedblock in non-std builds

* Get rid of `DoLookup` phantom.

* Attempt to make transaction_pool work with lookups.

* Remove vscode settings

* New attempt at making transaction pool work.

* It builds again!

* --all builds

* Fix tests.

* New build.

* Test account nonce reset.

* polkadot transaction pool tests/framework.

* Address grumbles.

* Revert bad `map_or`

* Rebuild binaries, workaround.

* Avoid casting to usize early.

* reenable sync tests

substrate/polkadot/api/src/lib.rs

@@ -26,6 +26,7 @@ extern crate substrate_client as client;
 extern crate substrate_executor as substrate_executor;
 extern crate substrate_runtime_executive;
 extern crate substrate_primitives;
+extern crate substrate_runtime_primitives as runtime_primitives;
 extern crate substrate_state_machine as state_machine;
 
 #[macro_use]
@@ -37,7 +38,9 @@ extern crate substrate_keyring as keyring;
 pub mod full;
 pub mod light;
 
-use primitives::{AccountId, Block, BlockId, Hash, Index, SessionKey, Timestamp, UncheckedExtrinsic};
+use primitives::{AccountId, Block, BlockId, Hash, Index, SessionKey, Timestamp,
+	UncheckedExtrinsic};
+use runtime::Address;
 use primitives::parachain::{CandidateReceipt, DutyRoster, Id as ParaId};
 
 error_chain! {
@@ -119,9 +122,12 @@ pub trait PolkadotApi {
 	/// Get the timestamp registered at a block.
 	fn timestamp(&self, at: &Self::CheckedBlockId) -> Result<Timestamp>;
 
-	/// Get the index of an account at a block.
+	/// Get the nonce (né index) of an account at a block.
 	fn index(&self, at: &Self::CheckedBlockId, account: AccountId) -> Result<Index>;
 
+	/// Get the account id of an address at a block.
+	fn lookup(&self, at: &Self::CheckedBlockId, address: Address) -> Result<Option<AccountId>>;
+
 	/// Get the active parachains at a block.
 	fn active_parachains(&self, at: &Self::CheckedBlockId) -> Result<Vec<ParaId>>;