Move two large error_reporting fn's to a separate file

[cengiz-io]

Hello!

I tried to make librustc/infer/error_reporting,rs more readable by modularizing it and moving its two largest functions to a separate file.

If you have any suggestions, please send it right away! 🚀

Thanks goes to @nikomatsakis for supporting.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @eddyb (or someone else) soon.

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[cengiz-io]

r? @nikomatsakis

[nikomatsakis]

@bors r+

[bors]

📌 Commit d21b0e3 has been approved by nikomatsakis

[nikomatsakis]

@bors r+

[bors]

📌 Commit 2f8ef50 has been approved by nikomatsakis

[nikomatsakis]

@bors r+

[bors]

📌 Commit e136bf6 has been approved by nikomatsakis