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/src/lib.rs

@@ -90,12 +90,11 @@ pub use crate::{
         RawEventDetails,
     },
     extrinsic::{
-        DefaultExtra,
-        DefaultExtraWithTxPayment,
         PairSigner,
-        SignedExtra,
-        Signer,
-        UncheckedExtrinsic,
+        PolkadotExtrinsicParams,
+        PolkadotExtrinsicParamsBuilder,
+        SubstrateExtrinsicParams,
+        SubstrateExtrinsicParamsBuilder,
     },
     metadata::{
         ErrorMetadata,