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>

substrate/frame/root-offences/src/lib.rs

@@ -27,6 +27,9 @@ mod mock;
 #[cfg(test)]
 mod tests;
 
+extern crate alloc;
+
+use alloc::vec::Vec;
 use pallet_session::historical::IdentificationTuple;
 use pallet_staking::{BalanceOf, Exposure, ExposureOf, Pallet as Staking};
 use sp_runtime::Perbill;
@@ -112,7 +115,7 @@ pub mod pallet {
 				.into_iter()
 				.map(|(o, _)| OffenceDetails::<T> {
 					offender: (o.clone(), Staking::<T>::eras_stakers(now, &o)),
-					reporters: vec![],
+					reporters: Default::default(),
 				})
 				.collect())
 		}