Bring rust-url's UserInfoFormatter implementation in-tree
Bring rust-url's UserInfoFormatter implementation in-tree

In version 1.0.0 of rust-url, the UserInfoFormatter has been removed. I
brought its implementation[1] over since it wasn't much code.

[1] -
https://github.com/servo/rust-url/commit/9e759f18726c8e1343162922b87163d4dd08fe3c#diff-0bb16ac13b75e9b568fa4aff61b0e71dL50

iron/iron,reem/iron,dorfsmay/iron