Remove sp_runtime::RuntimeString and replace with Cow<'static, str> or String depending on use case

[nazar-pc]

Description

As described in #4001 RuntimeVersion was not encoded consistently using serde. Turned out it was a remnant of old times and no longer actually needed. As such I removed it completely in this PR and replaced with Cow<'static, str> for spec/impl names and String for error cases.

Fixes #4001.

Integration

For downstream projects the upgrade will primarily consist of following two changes:

#[sp_version::runtime_version]
pub const VERSION: RuntimeVersion = RuntimeVersion {
-	spec_name: create_runtime_str!("statemine"),
-	impl_name: create_runtime_str!("statemine"),
+	spec_name: alloc::borrow::Cow::Borrowed("statemine"),
+	impl_name: alloc::borrow::Cow::Borrowed("statemine"),

		fn dispatch_benchmark(
			config: frame_benchmarking::BenchmarkConfig
-		) -> Result<Vec<frame_benchmarking::BenchmarkBatch>, sp_runtime::RuntimeString> {
+		) -> Result<Vec<frame_benchmarking::BenchmarkBatch>, alloc::string::String> {

SCALE encoding/decoding remains the same as before, but serde encoding in runtime has changed from bytes to string (it was like this in std environment already), which most projects shouldn't have issues with. I consider the impact of serde encoding here low due to the type only being used in runtime version struct and mostly limited to runtime internals, where serde encoding/decoding of this data structure is quite unlikely (though we did hit exactly this edge-case ourselves 😅).

Review Notes

Most of the changes are trivial and mechanical, the only non-trivial change is in substrate/primitives/version/proc-macro/src/decl_runtime_version.rs where macro call expectation in sp_version::runtime_version implementation was replaced with function call expectation.

Checklist

• My PR includes a detailed description as outlined in the "Description" and its two subsections above.
• My PR follows the labeling requirements of this project (at minimum one label for T required)
  • External contributors: ask maintainers to put the right label on your PR.
• I have made corresponding changes to the documentation (if applicable)

[nazar-pc]

I do not understand why formatter in CI wants those trailing whitespaces, I'm unable to commit them for some reason in IDE.

UPD: nano did it.

[bkchr]

create_runtime_str macro should stay and annotated as deprecated.

[nazar-pc]

I was thinking about that and to also make RuntimeString a type alias to the Cow, but wasn't sure if it was worth it. Seemed like a trivial change to upgrade, also required re-export of the hidden Cow from the crate, which I also didn't like at all, and if RuntimeString is removed then it is still a breaking change one way or the other.

If still desirable, should I also keep re-exports in sp-version?

[nazar-pc]

@bkchr I'd like to get clarity here so I can update PR with corresponding changes

[nazar-pc]

Fixed merge conflicts and restored create_runtime_str