Make sp_core and sp_runtime dependencies optional, and bump to latest (#760)

* begin porting over traits; remove Config use of Hash

* port over the Header bits that we need

* sp_core_hashing where possible, move Verify to PairSigner, remove unused errors

* tidy up Config things and move related bits into one place

* fix codegen

* copy Era over

* move AccountId, Address, Signer to Signer trait and a pass over fixing examples

* impl MultiAddress, MultiSignature, AccountId32 and add back to Config (for decoding later)

* Copy over StorageKey, StorageData, StorageChangeSet

* subxt core compiling with no sp_core or sp_runtime

* Get examples compiling

* pass over fixing tests

* cargo fmt

* clippy tweaks and update polkadot.rs

* fix codegen docs

* port over special DigestItem encoding/decoding

* clippy and doc fixes

* cargo fmt and example fix

* more cargo fmt-ing...

* substrate-extra to substrate-compat

* cargo.toml comments

* simplify PairSigner trait bounds

* move RPC types to a separate file

* fix docs

* Add some tests for things and other PR feedback

* bump to latest sp deps

* avoid needing substrate-compat feature in a test

examples/examples/fetch_staking_details.rs

@@ -10,15 +10,13 @@
 //! polkadot --dev --tmp
 //! ```
 
+use sp_core::{
+    sr25519,
+    Pair,
+};
 use sp_keyring::AccountKeyring;
 use subxt::{
-    ext::{
-        sp_core::{
-            sr25519,
-            Pair,
-        },
-        sp_runtime::AccountId32,
-    },
+    utils::AccountId32,
     OnlineClient,
     PolkadotConfig,
 };