Metadata V16: Be more dynamic over which hasher is used. (#1974)

* Use DynamicHasher256 to support Blake2 or Keccack depending on chain

* remove Config::Hash associated type, replace with HashFor<Config> alias

* Fix doc links

* fix wasm tests

* Don't strip system pallet associated types. check System.Hashing, not Hash. Rename BlockHash trait to Hash

* Tweak comment

* fmt

* fix merge

* Fix typo

core/src/client.rs

@@ -4,7 +4,10 @@
 
 //! A couple of client types that we use elsewhere.
 
-use crate::{config::Config, metadata::Metadata};
+use crate::{
+    config::{Config, HashFor},
+    metadata::Metadata,
+};
 use derive_where::derive_where;
 
 /// This provides access to some relevant client state in transaction extensions,
@@ -12,7 +15,7 @@ use derive_where::derive_where;
 #[derive_where(Clone, Debug)]
 pub struct ClientState<C: Config> {
     /// Genesis hash.
-    pub genesis_hash: C::Hash,
+    pub genesis_hash: HashFor<C>,
     /// Runtime version.
     pub runtime_version: RuntimeVersion,
     /// Metadata.