Reduce usage of Blake2Hasher (#5132)

This reduces the usage of `Blake2Hasher` in the code base and replaces
it with `BlakeTwo256`. The most important change is the removal of the
custom extern function for `Blake2Hasher`. The runtime `Hash` trait is
now also simplified and directly requires that the implementing type
implements `Hashable`.

substrate/primitives/core/src/lib.rs

@@ -61,6 +61,7 @@ pub mod ed25519;
 pub mod sr25519;
 pub mod ecdsa;
 pub mod hash;
+#[cfg(feature = "std")]
 mod hasher;
 pub mod offchain;
 pub mod sandbox;
@@ -77,8 +78,7 @@ pub use changes_trie::{ChangesTrieConfiguration, ChangesTrieConfigurationRange};
 pub use crypto::{DeriveJunction, Pair, Public};
 
 pub use hash_db::Hasher;
-// Switch back to Blake after PoC-3 is out
-// pub use self::hasher::blake::BlakeHasher;
+#[cfg(feature = "std")]
 pub use self::hasher::blake2::Blake2Hasher;
 
 pub use sp_storage as storage;