druntime redesign

[IGotD-]

Trying to adapt druntime has been a cascade of version (this) {} 
else version (that). druntime and phobos is a ball of wool.

After porting druntime I realized that I need Phobos as well 
because there is a lot of simple stuff like converting int to 
string and so on. Converting an int to string doesn't rely on an 
operating system. However, I'm forced to go through this version 
cascade over again which is even bigger that druntime.

The situation is similar to the C library which also might have 
dependencies all over. Some C library implementations are more 
simple and OS support can be stubbed. What D needs is a stubbed 
ABI for druntime/phobos. This means no file support, no threading 
etc. This ties into the "pay as you go" goal of D. D should be 
able to compile with minimal OS support instead of now require a 
rich OS.

What we need is an ABI. Then we also need a stubbed 
implementation of the ABI. This gives any implementer a good 
overview what needs to be implemented as well as that the OS 
support can be done incrementally. Some OSes don't support 
everything like memory mapped files and we should support this 
possibility as well.

We should also try to isolate the OS independent functionality 
more. Convenience functions that doesn't need an OS should be 
broken out, or at least with switches be compiled isolated from 
the OS dependent code.

If we are able to do this, D will make a rocket career in 
embedded systems.