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/scored-pool/src/mock.rs

@@ -20,7 +20,7 @@ use super::*;
 
 use std::cell::RefCell;
 use support::{impl_outer_origin, parameter_types};
-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 requried.
 use sr_primitives::{
@@ -132,7 +132,7 @@ impl Trait for Test {
 
 // This function basically just builds a genesis storage key/value store according to
 // our desired mockup.
-pub fn new_test_ext() -> runtime_io::TestExternalities<Blake2Hasher> {
+pub 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> {