Deprecate this crate when Rust 1.81 is released?

Rust 1.81 will stabilize the `error_in_core` feature: https://releases.rs/docs/1.81.0/#1810-beta

This will make the `Error` trait stable as `core::error::Error`: https://doc.rust-lang.org/beta/core/error/trait.Error.html

I believe this crate's raison d'etre was working around the unavailability of this trait in `no_std` contexts. Now it should be possible to use the `Error` trait ubiquitously regardless of if `std` is available or not.