[phobos] I applied @safe, @trusted, @system to Phobos!

[SHOO]

3. Bugzilla 4220 - I cannot apply @safe to intrinsic operation
     http://d.puremagic.com/issues/show_bug.cgi?id=4220
4. Bugzilla 4221 - dmd should not allow overload between @safe/@trusted 
and @system
     http://d.puremagic.com/issues/show_bug.cgi?id=4221

5. Bugzilla 4222 - druntime should apply @safe/@system/@trusted
     http://d.puremagic.com/issues/show_bug.cgi?id=4222

But, 6th and 7th are not reported. Because there is still room for 
argument in the 6th, and I didn't understand causes about the 7th 
problem well.

Walter Bright さんは書きました:
> These are great comments, please add each to their own bugzilla entry if 
> they aren't already.
>