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/Cargo.lock

@@ -6045,6 +6045,7 @@ dependencies = [
  "sp-externalities",
  "sp-io",
  "sp-panic-handler",
+ "sp-runtime",
  "sp-runtime-interface",
  "sp-serializer",
  "sp-state-machine",
@@ -7273,6 +7274,7 @@ dependencies = [
 name = "sp-runtime"
 version = "2.0.0-alpha.3"
 dependencies = [
+ "hash256-std-hasher",
  "impl-trait-for-tuples",
  "log 0.4.8",
  "parity-scale-codec",
@@ -7324,8 +7326,8 @@ name = "sp-runtime-interface-test"
 version = "2.0.0-dev"
 dependencies = [
  "sc-executor",
- "sp-core",
  "sp-io",
+ "sp-runtime",
  "sp-runtime-interface",
  "sp-runtime-interface-test-wasm",
  "sp-state-machine",
@@ -7397,6 +7399,7 @@ dependencies = [
  "sp-core",
  "sp-externalities",
  "sp-panic-handler",
+ "sp-runtime",
  "sp-trie",
  "trie-db",
  "trie-root",
@@ -7464,6 +7467,7 @@ dependencies = [
  "memory-db",
  "parity-scale-codec",
  "sp-core",
+ "sp-runtime",
  "sp-std",
  "trie-bench",
  "trie-db",