Refactor: extract functions and types for creating extrinsics (#138)

* Extract extrinsic

* Extract extrinsic types to own module

* Fix compiler errors

* Fix warnings

* Fmt

* Pin wabt-sys to 0.7.1, it fails (on my machine) with 0.7.2

* Fix up tests

* Fmt

* Reexport RuntimeVersion

src/frame/balances.rs

@@ -115,7 +115,7 @@ mod tests {
             RuntimeError,
         },
         events::EventsDecoder,
-        signer::{
+        extrinsic::{
             PairSigner,
             Signer,
         },

src/frame/sudo.rs

@@ -44,8 +44,8 @@ mod tests {
     use super::*;
     use crate::{
         error::Error,
+        extrinsic::PairSigner,
         frame::balances::TransferCall,
-        signer::PairSigner,
         tests::{
             test_client,
             TestRuntime,