Offchain-worker APIs stubs (#2615)

* WiP: HTTP Apis.

* Working on the API.

* Add docs, clean up the API.

* Expose ext_ stuff as well.

* Implement HTTP helpers for offchain sr-io.

* Remove HTTP stuff.

* Revert "Remove HTTP stuff."

This reverts commit 7cca029d6ae93c5849b50edfcc6d2c313ba3e5bf.

* HTTP apis.

* Additional offchain methods.

* Make it compile.

* Implement wasm-ext boundary of offchain methods.

* Add stubs for offchain stuff to prevent panics.

* Fix tests.

* Addres some more issues.

* Introduce typedef, use unsafe from_utf8

* Bump runtime version.

* Introduce error to distinguish deadline and io errors.

* Add local_storage_cas

* Some tests for offchain stuff.

* Address more grumbles.

* Fix tests compilation.

* Fix borked merge.

* Improve docs for expected return values from ext functions.

* Adding new sign/enrypt/decrypt APIs.

substrate/core/state-machine/src/basic.rs

@@ -20,6 +20,7 @@ use std::collections::HashMap;
 use std::iter::FromIterator;
 use hash_db::Hasher;
 use trie::trie_root;
+use primitives::offchain;
 use primitives::storage::well_known_keys::{CHANGES_TRIE_CONFIG, CODE, HEAP_PAGES};
 use parity_codec::Encode;
 use super::{ChildStorageKey, Externalities, OverlayedChanges};
@@ -155,9 +156,9 @@ impl<H: Hasher> Externalities<H> for BasicExternalities where H::Out: Ord {
 		Ok(None)
 	}
 
-	fn submit_extrinsic(&mut self, _extrinsic: Vec<u8>) -> Result<(), ()> {
-		warn!("Call to submit_extrinsic without offchain externalities set.");
-		Err(())
+	fn offchain(&mut self) -> Option<&mut offchain::Externalities> {
+		warn!("Call to non-existent out offchain externalities set.");
+		None
 	}
 }