Remove `error_in_core` feature

[haerdib]

Error should now be implemented fully in core, so there should be no need for this feature anymore.

• Remove https://github.com/scs/substrate-api-client/blob/master/src/lib.rs#L18
• Remove distinction between std::error and core::error (e.g. remove https://github.com/scs/substrate-api-client/blob/master/src/api/error.rs#L26-L29)

Related issue: https://github.com/rust-lang/rust/issues/103765

[Niederb]

If I remember correctly this is the last feature that prevents us from switching to a stable Rust version. So we could try switching to stable.

[haerdib]

Good idea, feature will be stabilized in 2 days (on the 5th September). So let's wait until then and see if we can use the stable toolchain.

[haerdib]

Switchin to stable is currently not possible (or requires quite some effort) because of our no_std check. There, we require the following features:

#![feature(start, libc, lang_items)]
#![feature(alloc_error_handler)]

At least for the alloc_error_handler there does not seem to be an easy alternative: https://github.com/rust-lang/rust/issues/51540

I did a quick research to see if there's a better alternative for no_stds check but I didn't really find one. cargo-nono did not install with cargo install cargo-nono and it was updated last 4 years ago, it doesn't really seem a good alternative.

So I think it's currenlty not worth it to switch to stable.

[masapr]

So we should move the feature to the icebox?

[haerdib]

No, the error_in_core feature can still be removed. Only the switch to stable toolchain is not easily possible. PR https://github.com/scs/substrate-api-client/pull/803 will close this issue.