CTFE writeln again (__ctfeWriteln)

[KennyTM~]

Pull request: https://github.com/D-Programming-Language/dmd/pull/296

Previous discussion: 
http://www.digitalmars.com/d/archives/digitalmars/D/CTFE_writeln_140241.html

This is the 2nd try to add a compile-time printing facility to D. The 
previous pull request uses std.stdio.writeln which causes some concern, 
and then left unattended for a few weeks after the last comment. So I go 
for my 2nd alternative, which is to add a magic function __ctfeWriteln 
that does the same.

In my implementation, __ctfeWriteln will print the interpreted arguments 
to stdmsg when CTFE-ed, and is a no-op in runtime (it can be configured 
to throw an exception or actually print the arguments or anything you like).