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>

polkadot/primitives/Cargo.toml

@@ -26,7 +26,7 @@ sp-arithmetic = { path = "../../substrate/primitives/arithmetic", default-featur
 sp-authority-discovery = { path = "../../substrate/primitives/authority-discovery", default-features = false, features = ["serde"] }
 sp-consensus-slots = { path = "../../substrate/primitives/consensus/slots", default-features = false, features = ["serde"] }
 sp-io = { path = "../../substrate/primitives/io", default-features = false }
-sp-keystore = { path = "../../substrate/primitives/keystore", optional = true }
+sp-keystore = { path = "../../substrate/primitives/keystore", optional = true, default-features = false }
 sp-staking = { path = "../../substrate/primitives/staking", default-features = false, features = ["serde"] }
 sp-std = { package = "sp-std", path = "../../substrate/primitives/std", default-features = false }
 
@@ -53,6 +53,7 @@ std = [
 	"sp-consensus-slots/std",
 	"sp-io/std",
 	"sp-keystore",
+	"sp-keystore?/std",
 	"sp-staking/std",
 	"sp-std/std",
 ]