First step for generating host externals out of the function definition in sr-io (#3567)

* Adds new wrapper traits for wasm executor

* Add new crate `substrate-wasm-interface`

Thew new crate holds types and traits for the communicating between the
wasm runtime and the host.

* Rewrite externals with new macro etc

* Fix vec initialization

* Make executor tests working

* Remove unused code + warnings

* Introduce `Pointer` and `WordSize` for working with wasm

* Fix tests and compilation

* Fix compilation

* Apply suggestions from code review

Co-Authored-By: Sergei Pepyakin <sergei@parity.io>

* Review feedback

* Remove unused conversions

* Make each host function its own struct

`HostFunctions` now just returns these function structs. Each function
can be executed by using one of the function structs. The inherent host
functions are now moved to the "normal" host functions.

* Remove byteorder

* Add floating point types

* Make pointer interface more safe

* Add type alias for wasm-interface Result

* More review comments

substrate/srml/system/src/offchain.rs

@@ -82,7 +82,7 @@ pub trait SubmitSignedTransaction<T: crate::Trait, Call> {
 			::create_transaction::<Self::Signer>(call, id, expected)
 			.ok_or(())?;
 		let xt = Self::Extrinsic::new(call, Some(signature_data)).ok_or(())?;
-		runtime_io::submit_transaction(&xt)
+		runtime_io::submit_transaction(xt.encode())
 	}
 }
 
@@ -97,7 +97,7 @@ pub trait SubmitUnsignedTransaction<T: crate::Trait, Call> {
 	/// and `Err` if transaction was rejected from the pool.
 	fn submit_unsigned(call: impl Into<Call>) -> Result<(), ()> {
 		let xt = Self::Extrinsic::new(call.into(), None).ok_or(())?;
-		runtime_io::submit_transaction(&xt)
+		runtime_io::submit_transaction(xt.encode())
 	}
 }