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

subxt/src/lib.rs

@@ -57,10 +57,10 @@ pub mod utils;
 /// Polkadot node.
 pub mod config {
     pub use subxt_core::config::{
-        polkadot, substrate, transaction_extensions, BlockHash, Config, DefaultExtrinsicParams,
-        DefaultExtrinsicParamsBuilder, ExtrinsicParams, ExtrinsicParamsEncoder, Hasher, Header,
-        PolkadotConfig, PolkadotExtrinsicParams, SubstrateConfig, SubstrateExtrinsicParams,
-        TransactionExtension,
+        polkadot, substrate, transaction_extensions, Config, DefaultExtrinsicParams,
+        DefaultExtrinsicParamsBuilder, ExtrinsicParams, ExtrinsicParamsEncoder, Hash, HashFor,
+        Hasher, Header, PolkadotConfig, PolkadotExtrinsicParams, SubstrateConfig,
+        SubstrateExtrinsicParams, TransactionExtension,
     };
     pub use subxt_core::error::ExtrinsicParamsError;
 }