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

README.md

@@ -38,13 +38,13 @@ resides ([`CARGO_MANIFEST_DIR`](https://doc.rust-lang.org/cargo/reference/enviro
 ### Initializing the API client
 
 ```rust
-use subxt::{ClientBuilder, DefaultConfig, DefaultExtra};
+use subxt::{ClientBuilder, DefaultConfig, SubstrateExtrinsicParams};
 
 let api = ClientBuilder::new()
     .set_url("wss://rpc.polkadot.io:443")
     .build()
     .await?
-    .to_runtime_api::<node_runtime::RuntimeApi<DefaultConfig, DefaultExtra<DefaultConfig>>>();
+    .to_runtime_api::<node_runtime::RuntimeApi<DefaultConfig, SubstrateExtrinsicParams<DefaultConfig>>>();
 ```
 
 The `RuntimeApi` type is generated by the `subxt` macro from the supplied metadata. This can be parameterized with user
@@ -53,7 +53,7 @@ chain.
 
 ### Querying Storage
 
-Call the generated `RuntimeApi::storage()` method, followed by the `pallet_name()` and then the `storage_item_name()`. 
+Call the generated `RuntimeApi::storage()` method, followed by the `pallet_name()` and then the `storage_item_name()`.
 
 So in order to query `Balances::TotalIssuance`: