Introduce Metadata type (#974)

* WIP new Metadata type

* Finish basic Metadata impl inc hashing and validation

* remove caching from metadata; can add that higher up

* remove caches

* update retain to use Metadata

* clippy fixes

* update codegen to use Metadata

* clippy

* WIP fixing subxt lib

* WIP fixing tests, rebuild artifacts, fix OrderedMap::retain

* get --all-targets compiling

* move DispatchError type lookup back to being optional

* cargo clippy

* fix docs

* re-use VariantIndex to get variants

* add docs and enforce docs on metadata crate

* fix docs

* add test and fix docs

* cargo fmt

* address review comments

* update lockfiles

* ExactSizeIter so we can ask for len() of things (and hopefully soon is_empty()

examples/examples/runtime_apis_dynamic.rs

@@ -14,7 +14,8 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     // `AccountNonceApi_account_nonce` function.
     let account = AccountKeyring::Alice.to_account_id();
     let runtime_api_call = subxt::dynamic::runtime_api_call(
-        "AccountNonceApi_account_nonce",
+        "AccountNonceApi",
+        "account_nonce",
         vec![Value::from_bytes(account)],
     );
 

examples/examples/setup_client_offline.rs

@@ -1,5 +1,4 @@
 use subxt::ext::codec::Decode;
-use subxt::ext::frame_metadata::RuntimeMetadataPrefixed;
 use subxt::metadata::Metadata;
 use subxt::utils::H256;
 use subxt::{config::PolkadotConfig, OfflineClient};
@@ -26,8 +25,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     //    `subxt metadata > file.scale` to download it):
     let metadata = {
         let bytes = std::fs::read("./artifacts/polkadot_metadata_small.scale").unwrap();
-        let metadata = RuntimeMetadataPrefixed::decode(&mut &*bytes).unwrap();
-        Metadata::try_from(metadata).unwrap()
+        Metadata::decode(&mut &*bytes).unwrap()
     };
 
     // Create an offline client using the details obtained above: