[phobos] Changes to std.complex

[Lars Tandle Kyllingstad]

As you may have noticed, I have opened a pull request for adding sin(),
cos() and sqrt() to std.complex.  Don, since you are the maintainer of
std.math, it would be great if you could take a look at it and give your
approval (or disapproval).

I was also considering making abs(), arg() and conj() free functions
rather than members of Complex!T.  This would be consistent with
std.math, and it will help ease the transition to library complex
numbers.  For abs() in particular, it will also help with generic code.
Any objections to this?

-Lars