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/offchain.rs

@@ -17,8 +17,7 @@
 //! Offchain workers types
 
 use codec::{Encode, Decode};
-use rstd::prelude::{Vec, Box};
-use rstd::convert::TryFrom;
+use rstd::{prelude::{Vec, Box}, convert::TryFrom};
 
 pub use crate::crypto::KeyTypeId;
 
@@ -663,110 +662,17 @@ impl<T: Externalities> Externalities for LimitedExternalities<T> {
 	}
 }
 
-/// An implementation of offchain extensions that should never be triggered.
-pub enum NeverOffchainExt {}
-
-impl NeverOffchainExt {
-	/// Create new offchain extensions.
-	pub fn new<'a>() -> Option<&'a mut Self> {
-		None
-	}
+#[cfg(feature = "std")]
+externalities::decl_extension! {
+	/// The offchain extension that will be registered at the Substrate externalities.
+	pub struct OffchainExt(Box<dyn Externalities>);
 }
 
-impl Externalities for NeverOffchainExt {
-	fn is_validator(&self) -> bool {
-		unreachable!()
-	}
-
-	fn submit_transaction(&mut self, _extrinsic: Vec<u8>) -> Result<(), ()> {
-		unreachable!()
-	}
-
-	fn network_state(
-		&self,
-	) -> Result<OpaqueNetworkState, ()> {
-		unreachable!()
-	}
-
-	fn timestamp(&mut self) -> Timestamp {
-		unreachable!()
-	}
-
-	fn sleep_until(&mut self, _deadline: Timestamp) {
-		unreachable!()
-	}
-
-	fn random_seed(&mut self) -> [u8; 32] {
-		unreachable!()
-	}
-
-	fn local_storage_set(&mut self, _kind: StorageKind, _key: &[u8], _value: &[u8]) {
-		unreachable!()
-	}
-
-	fn local_storage_compare_and_set(
-		&mut self,
-		_kind: StorageKind,
-		_key: &[u8],
-		_old_value: Option<&[u8]>,
-		_new_value: &[u8],
-	) -> bool {
-		unreachable!()
-	}
-
-	fn local_storage_get(&mut self, _kind: StorageKind, _key: &[u8]) -> Option<Vec<u8>> {
-		unreachable!()
-	}
-
-	fn http_request_start(
-		&mut self,
-		_method: &str,
-		_uri: &str,
-		_meta: &[u8]
-	) -> Result<HttpRequestId, ()> {
-		unreachable!()
-	}
-
-	fn http_request_add_header(
-		&mut self,
-		_request_id: HttpRequestId,
-		_name: &str,
-		_value: &str
-	) -> Result<(), ()> {
-		unreachable!()
-	}
-
-	fn http_request_write_body(
-		&mut self,
-		_request_id: HttpRequestId,
-		_chunk: &[u8],
-		_deadline: Option<Timestamp>
-	) -> Result<(), HttpError> {
-		unreachable!()
-	}
-
-	fn http_response_wait(
-		&mut self,
-		_ids: &[HttpRequestId],
-		_deadline: Option<Timestamp>
-	) -> Vec<HttpRequestStatus> {
-		unreachable!()
-	}
-
-	fn http_response_headers(
-		&mut self,
-		_request_id: HttpRequestId
-	) -> Vec<(Vec<u8>, Vec<u8>)> {
-		unreachable!()
-	}
-
-	fn http_response_read_body(
-		&mut self,
-		_request_id: HttpRequestId,
-		_buffer: &mut [u8],
-		_deadline: Option<Timestamp>
-	) -> Result<usize, HttpError> {
-		unreachable!()
+#[cfg(feature = "std")]
+impl OffchainExt {
+	/// Create a new instance of `Self`.
+	pub fn new<O: Externalities + 'static>(offchain: O) -> Self {
+		Self(Box::new(offchain))
 	}
 }