Implement core::error::Error for ParsingError

[jlxip]

This continues the discussion in #27.

Regarding cole14's last comment, this does work in my use case. However, implementing the trait core::error::Error instead of std::error::Error appears to be an experimental feature, which can only be used with nightly Rust.

For this reason, I've taken the liberty to add a new optional and non-default feature to the crate, nightly, since apparently it's not possible to deduce the release channel at compile time. Now:

• If the std feature is enabled, the original code is compiled.
• If the std feature is not enabled and the nightly feature is enabled, the experimental feature #![feature(error_in_core)] is used, and a similar† branch of the original code is compiled, one that substitutes std with core and does not match ParseError::IOError, which is only available with std.

† Maybe there's a better way to do this? I haven't looked extensively at the Rust preprocessor. Is there a #define-like capability?

[cole14]

Awesome, thanks for the background link to the progress towards Error being available from libcore on nightly. Looks like that got merged in for 1.65.0 which landed a few weeks after I added the no_std support to this crate. :)

I think the approach here looks fine to me and the duplicated code for different features seems to be a common pattern for these sorts of things.

[jlxip]

Cool. Merge whenever you're ready then. Thank you :)

[cole14]

Thanks again for the report and for the PR!

I want to say here in case you're wondering, that I'm going to wait a few weeks before I bundle this into an new crate version just in case further changes come in after this. My hope is to try and avoid churning crate versions too quickly. It sounds like that's probably fine for your use-case, but if you'd like me to push an updated crate with these changes sooner than that let me know.

[jlxip]

Oh, it's fine. Thanks a lot for the offer though. I'm using the git version and it works like a charm :)