Add Result::unwrap_infallible
Add Result::unwrap_infallible

Implementation of
https://github.com/rust-lang/rfcs/pull/2799

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust