Replace `std::error` with `core::error`

[StackOverflowExcept1on]

ring/src/error.rs

Lines 86 to 87 in 525047a

	#[cfg(feature = "std")]
	impl std::error::Error for Unspecified {}

rust-lang/rust#125951

[briansmith]

We can add a core-error feature flag to ring that will do this, I suppose. Not sure what the use case is.

[StackOverflowExcept1on]

Well, maybe use case is to not depend on std and be able to use crates like thiserror

[briansmith]

This idea seems fine with me, but it'd need to be behind a feature flag because we aren't raising the MSRV yet.