Add corefines

[jirutka]

https://github.com/jirutka/corefines
https://rubygems.org/gems/corefines

Corefines is a collection of general purpose refinements for extending the core capabilities of Ruby’s built-in classes. It’s like Facets or ActiveSupport, but doesn’t pollute the global scope (when run on Ruby that supports refinements).

However, it’s not just about core extensions. Corefines also provides some “syntactic sugar” for more convenient using of refinements and compatibility mode for rubies that don’t support refinements (yet)!

It’s used in production: cvut/sirius, asciidoctor-doctest.