Generate core::error::Error for no_std

[greyblake]

Rust 1.81.0 stabilizes core::error::Error: https://blog.rust-lang.org/2024/09/05/Rust-1.81.0.html#whats-in-1810-stable

With that we should able to remove the condition here: https://github.com/greyblake/nutype/blob/master/nutype_macros/src/common/gen/error.rs#L14-L31

And generate core::error::Error instead of ::std::error::Error.

Since it's a breaking (the older versions of Rust will not be properly supported), add a [BREAKING] note change to the CHANGELOG.