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/core/primitives/src/lib.rs

@@ -54,16 +54,15 @@ pub mod crypto;
 
 pub mod u32_trait;
 
-pub mod child_storage_key;
 pub mod ed25519;
 pub mod sr25519;
 pub mod hash;
 mod hasher;
 pub mod offchain;
 pub mod sandbox;
-pub mod storage;
 pub mod uint;
 mod changes_trie;
+#[cfg(feature = "std")]
 pub mod traits;
 pub mod testing;
 
@@ -81,6 +80,8 @@ pub use hash_db::Hasher;
 // pub use self::hasher::blake::BlakeHasher;
 pub use self::hasher::blake2::Blake2Hasher;
 
+pub use primitives_storage as storage;
+
 /// Context for executing a call into the runtime.
 pub enum ExecutionContext {
 	/// Context for general importing (including own blocks).