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/system/benches/bench.rs

@@ -17,9 +17,10 @@
 use criterion::{Criterion, criterion_group, criterion_main, black_box};
 use srml_system as system;
 use support::{decl_module, decl_event, impl_outer_origin, impl_outer_event};
-use runtime_io::with_externalities;
-use primitives::{H256, Blake2Hasher};
-use sr_primitives::{Perbill, traits::{BlakeTwo256, IdentityLookup}, testing::Header};
+use primitives::H256;
+use sr_primitives::{
+	set_and_run_with_externalities, Perbill, traits::{BlakeTwo256, IdentityLookup}, testing::Header,
+};
 
 mod module {
 	use super::*;
@@ -82,13 +83,13 @@ impl module::Trait for Runtime {
 	type Event = Event;
 }
 
-fn new_test_ext() -> runtime_io::TestExternalities<Blake2Hasher> {
+fn new_test_ext() -> runtime_io::TestExternalities {
 	system::GenesisConfig::default().build_storage::<Runtime>().unwrap().into()
 }
 
 fn deposit_events(n: usize) {
 	let mut t = new_test_ext();
-	with_externalities(&mut t, || {
+	set_and_run_with_externalities(&mut t, || {
 		for _ in 0..n {
 			module::Module::<Runtime>::deposit_event(
 				module::Event::Complex(vec![1, 2, 3], 2, 3, 899)