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()

subxt/src/client/offline_client.rs

@@ -73,13 +73,13 @@ impl<T: Config> OfflineClient<T> {
     pub fn new(
         genesis_hash: T::Hash,
         runtime_version: RuntimeVersion,
-        metadata: Metadata,
+        metadata: impl Into<Metadata>,
     ) -> OfflineClient<T> {
         OfflineClient {
             inner: Arc::new(Inner {
                 genesis_hash,
                 runtime_version,
-                metadata,
+                metadata: metadata.into(),
             }),
         }
     }