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/hasher.rs

@@ -16,27 +16,10 @@
 
 //! Substrate Blake2b Hasher implementation
 
-use hash_db::Hasher;
-use hash256_std_hasher::Hash256StdHasher;
-use crate::hash::H256;
-
 pub mod blake2 {
-	use super::{Hasher, Hash256StdHasher, H256};
-	#[cfg(feature = "std")]
-	use crate::hashing::blake2_256;
-
-	#[cfg(not(feature = "std"))]
-	extern "C" {
-		fn ext_blake2_256(data: *const u8, len: u32, out: *mut u8);
-	}
-	#[cfg(not(feature = "std"))]
-	fn blake2_256(data: &[u8]) -> [u8; 32] {
-		let mut result: [u8; 32] = Default::default();
-		unsafe {
-			ext_blake2_256(data.as_ptr(), data.len() as u32, result.as_mut_ptr());
-		}
-		result
-	}
+	use hash_db::Hasher;
+	use hash256_std_hasher::Hash256StdHasher;
+	use crate::hash::H256;
 
 	/// Concrete implementation of Hasher using Blake2b 256-bit hashes
 	#[derive(Debug)]
@@ -46,8 +29,9 @@ pub mod blake2 {
 		type Out = H256;
 		type StdHasher = Hash256StdHasher;
 		const LENGTH: usize = 32;
+
 		fn hash(x: &[u8]) -> Self::Out {
-			blake2_256(x).into()
+			crate::hashing::blake2_256(x).into()
 		}
 	}
 }

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;