Extract frame_system SignedExtensions into separate files. (#6474)

* Split the code.

* Restructure.

* Split tests.

* Self-review.

* Break lines.

* Move tests out.

* Rename CheckEra -> CheckMortality but keep backwards compatibility

* Update frame/system/src/extensions/check_mortality.rs

* Don't rename the IDENTIFIER for now.

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/frame/system/src/extensions/check_spec_version.rs

@@ -0,0 +1,58 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2017-2020 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use crate::{Trait, Module};
+use codec::{Encode, Decode};
+use sp_runtime::{
+	traits::SignedExtension,
+	transaction_validity::TransactionValidityError,
+};
+
+/// Ensure the runtime version registered in the transaction is the same as at present.
+#[derive(Encode, Decode, Clone, Eq, PartialEq)]
+pub struct CheckSpecVersion<T: Trait + Send + Sync>(sp_std::marker::PhantomData<T>);
+
+impl<T: Trait + Send + Sync> sp_std::fmt::Debug for CheckSpecVersion<T> {
+	#[cfg(feature = "std")]
+	fn fmt(&self, f: &mut sp_std::fmt::Formatter) -> sp_std::fmt::Result {
+		write!(f, "CheckSpecVersion")
+	}
+
+	#[cfg(not(feature = "std"))]
+	fn fmt(&self, _: &mut sp_std::fmt::Formatter) -> sp_std::fmt::Result {
+		Ok(())
+	}
+}
+
+impl<T: Trait + Send + Sync> CheckSpecVersion<T> {
+	/// Create new `SignedExtension` to check runtime version.
+	pub fn new() -> Self {
+		Self(sp_std::marker::PhantomData)
+	}
+}
+
+impl<T: Trait + Send + Sync> SignedExtension for CheckSpecVersion<T> {
+	type AccountId = T::AccountId;
+	type Call = <T as Trait>::Call;
+	type AdditionalSigned = u32;
+	type Pre = ();
+	const IDENTIFIER: &'static str = "CheckSpecVersion";
+
+	fn additional_signed(&self) -> Result<Self::AdditionalSigned, TransactionValidityError> {
+		Ok(<Module<T>>::runtime_version().spec_version)
+	}
+}