Please do not put this in `std.range` or even `std.container`. Very rarely does a module need a circular buffer. The module situation is bad enough as it is, let's not make it worse.

Put it in `std.circularbuffer`. That way, this module can import just `CircularBuffer` and not the whole of Phobos. Hopefully at some point we can revert the mistake of having a `std.container` module, which was destined from the get-go to be a monster containing every conceivable data structure. Phobos really needs to be more organised and we might as well start now.