Simplify creating and signing extrinsics (#490)

* WIP extrinsic api updates

* First pass done; now to get things compiling again

* document and tweak new structs/traits

* cargo check --all-targets now compiles without issue

* Polkadot and Substrate take different extra params; support both

* Fix transaction format (missing byte from AccountId -> Address) and fmt

* Tweak Signer trait

* Tweak comments and such in extrinsic params

* check all examples against newer polkadot, add new one with params, import path tweaks

* clippy fix, and save an allocation when signing

* Remove unnecessary Default clauses

* Tidy up and fix comments. Panic if payload size >4GB

* fix typo

subxt/tests/integration/frame/system.rs

@@ -24,7 +24,6 @@ use crate::{
 };
 use assert_matches::assert_matches;
 use sp_keyring::AccountKeyring;
-use subxt::Signer;
 
 #[async_std::test]
 async fn storage_account() -> Result<(), subxt::Error<DispatchError>> {
@@ -52,7 +51,7 @@ async fn tx_remark_with_event() -> Result<(), subxt::Error<DispatchError>> {
         .tx()
         .system()
         .remark_with_event(b"remarkable".to_vec())
-        .sign_and_submit_then_watch(&alice)
+        .sign_and_submit_then_watch_default(&alice)
         .await?
         .wait_for_finalized_success()
         .await?