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/src/lib.rs

@@ -34,9 +34,9 @@
 //!
 //! See [`polkadot_sdk::frame`](../polkadot_sdk_docs/polkadot_sdk/frame_runtime/index.html).
 //!
-//! ## Warning: Experimental
+//! ## WARNING: Experimental
 //!
-//! This crate and all of its content is experimental, and should not yet be used in production.
+//! **This crate and all of its content is experimental, and should not yet be used in production.**
 //!
 //! ## Underlying dependencies
 //!