[D-runtime] Update request from tango runtime

[SiegeLord]

I don't use the feature myself, but a few parts of Tango do use it 
(primarily the IO classes). I took a cursory look at how it is used and 
as best as I can tell, most functions are used. Some unused functions 
seem to be 'ready' and 'spawn'.

I also looked into make a derived Fiber class with those features, but 
some functions (e.g. switchOut) are final...

-SiegeLord