Move Externalities into its own crate (#3775)

* Move `Externalities` into `substrate-externalities`

- `Externalities` now support generic extensions
- Split of `primtives-storage` for storage primitive types

* Move the externalities scoping into `substrate-externalities`

* Fix compilation

* Review feedback

* Adds macro for declaring extensions

* Fix benchmarks

* Introduce `ExtensionStore` trait

* Last review comments

* Implement it for `ExtensionStore`

substrate/srml/example/src/lib.rs

@@ -634,15 +634,12 @@ mod tests {
 	use super::*;
 
 	use support::{assert_ok, impl_outer_origin, parameter_types};
-	use runtime_io::with_externalities;
-	use primitives::{H256, Blake2Hasher};
+	use primitives::H256;
 	// The testing primitives are very useful for avoiding having to work with signatures
 	// or public keys. `u64` is used as the `AccountId` and no `Signature`s are required.
 	use sr_primitives::{
-		Perbill,
+		set_and_run_with_externalities, Perbill, weights::GetDispatchInfo, testing::Header,
 		traits::{BlakeTwo256, OnInitialize, OnFinalize, IdentityLookup},
-		weights::GetDispatchInfo,
-		testing::Header
 	};
 
 	impl_outer_origin! {
@@ -707,7 +704,7 @@ mod tests {
 
 	// This function basically just builds a genesis storage key/value store according to
 	// our desired mockup.
-	fn new_test_ext() -> runtime_io::TestExternalities<Blake2Hasher> {
+	fn new_test_ext() -> runtime_io::TestExternalities {
 		let mut t = system::GenesisConfig::default().build_storage::<Test>().unwrap();
 		// We use default for brevity, but you can configure as desired if needed.
 		balances::GenesisConfig::<Test>::default().assimilate_storage(&mut t).unwrap();
@@ -722,7 +719,7 @@ mod tests {
 
 	#[test]
 	fn it_works_for_optional_value() {
-		with_externalities(&mut new_test_ext(), || {
+		set_and_run_with_externalities(&mut new_test_ext(), || {
 			// Check that GenesisBuilder works properly.
 			assert_eq!(Example::dummy(), Some(42));
 
@@ -743,7 +740,7 @@ mod tests {
 
 	#[test]
 	fn it_works_for_default_value() {
-		with_externalities(&mut new_test_ext(), || {
+		set_and_run_with_externalities(&mut new_test_ext(), || {
 			assert_eq!(Example::foo(), 24);
 			assert_ok!(Example::accumulate_foo(Origin::signed(1), 1));
 			assert_eq!(Example::foo(), 25);
@@ -752,7 +749,7 @@ mod tests {
 
 	#[test]
 	fn signed_ext_watch_dummy_works() {
-		with_externalities(&mut new_test_ext(), || {
+		set_and_run_with_externalities(&mut new_test_ext(), || {
 			let call = <Call<Test>>::set_dummy(10);
 			let info = DispatchInfo::default();