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/runtime/Cargo.toml

@@ -24,6 +24,7 @@ rand = { version = "0.7.2", optional = true }
 impl-trait-for-tuples = "0.1.3"
 sp-inherents = { version = "2.0.0-alpha.2", default-features = false, path = "../inherents" }
 parity-util-mem = { version = "0.5.1", default-features = false, features = ["primitive-types"] }
+hash256-std-hasher = { version = "0.15.2", default-features = false }
 
 [dev-dependencies]
 serde_json = "1.0.41"
@@ -44,4 +45,5 @@ std = [
 	"serde",
 	"sp-inherents/std",
 	"parity-util-mem/std",
+	"hash256-std-hasher/std",
 ]