std.range.cacheFront proposal&working code: wraps a range to enforce front is called only once

[deadalnix]

Overall, I think it is greatly needed :D

You should propose that as a pull request. Also, you shouldn't 
alias this the source range, as thing now become impracticable.

IMO, CacheFront should be a froward range whatever the source is 
and only provide front/popFront.