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/indices/Cargo.toml

@@ -23,7 +23,7 @@ frame-support = { path = "../support", default-features = false }
 frame-system = { path = "../system", default-features = false }
 sp-core = { path = "../../primitives/core", default-features = false }
 sp-io = { path = "../../primitives/io", default-features = false }
-sp-keyring = { path = "../../primitives/keyring", optional = true }
+sp-keyring = { path = "../../primitives/keyring", optional = true, default-features = false }
 sp-runtime = { path = "../../primitives/runtime", default-features = false }
 sp-std = { path = "../../primitives/std", default-features = false }
 
@@ -42,6 +42,7 @@ std = [
 	"sp-core/std",
 	"sp-io/std",
 	"sp-keyring",
+	"sp-keyring?/std",
 	"sp-runtime/std",
 	"sp-std/std",
 ]