Import std.functional.unaryFun, which got accidentally removed during the cherry-pick

Import std.functional.unaryFun, which got accidentally removed during the cherry-pick

Revive https://github.com/D-Programming-Language/phobos/pull/1348 as it has been abondoned by the author.
Merge branch 'std-range-tee' of https://github.com/MetaLang/phobos into std-range-tee