Fix nostd build of several crates (#4060)

Preparation for https://github.com/paritytech/polkadot-sdk/pull/3935

Changes:
- Add some `default-features = false` for the case that a crate and that
dependency both support nostd builds.
- Shuffle files around of some benchmarking-only crates. These
conditionally disabled the `cfg_attr` for nostd and pulled in libstd.
Example [here](https://github.com/ggwpez/zepter/pull/95). The actual
logic is moved into a `inner.rs` to preserve nostd capability of the
crate in case the benchmarking feature is disabled.
- Add some `use sp_std::vec` where needed.
- Remove some `optional = true` in cases where it was not optional.
- Removed one superfluous `cfg_attr(not(feature = "std"), no_std..`.

All in all this should be logical no-op.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

cumulus/pallets/session-benchmarking/src/inner.rs

@@ -0,0 +1,42 @@
+// Copyright (C) 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.
+
+//! Benchmarking setup for pallet-session.
+
+use sp_std::{prelude::*, vec};
+
+use frame_benchmarking::{benchmarks, whitelisted_caller};
+use frame_system::RawOrigin;
+use pallet_session::*;
+use parity_scale_codec::Decode;
+pub struct Pallet<T: Config>(pallet_session::Pallet<T>);
+pub trait Config: pallet_session::Config {}
+
+benchmarks! {
+	set_keys {
+		let caller: T::AccountId = whitelisted_caller();
+		frame_system::Pallet::<T>::inc_providers(&caller);
+		let keys = T::Keys::decode(&mut sp_runtime::traits::TrailingZeroInput::zeroes()).unwrap();
+		let proof: Vec<u8> = vec![0,1,2,3];
+	}: _(RawOrigin::Signed(caller), keys, proof)
+
+	purge_keys {
+		let caller: T::AccountId = whitelisted_caller();
+		frame_system::Pallet::<T>::inc_providers(&caller);
+		let keys = T::Keys::decode(&mut sp_runtime::traits::TrailingZeroInput::zeroes()).unwrap();
+		let proof: Vec<u8> = vec![0,1,2,3];
+		let _t = pallet_session::Pallet::<T>::set_keys(RawOrigin::Signed(caller.clone()).into(), keys, proof);
+	}: _(RawOrigin::Signed(caller))
+}