Refactor code with Rust 1.35.0 introduced features

[cipriancraciun]

Based on https://blog.rust-lang.org/2019/05/23/Rust-1.35.0.html, update the code where needed to use the newly introduced:

• Box<dyn Fn*>;
• Ref::map_split;
• RefCell::replace_with;
• ptr::hash;