Library standardization

[Walter Bright]

Edward Diener wrote:
> In D one of the negatives of having everything in a single .d file is 
> that there is no separation between the declaration and the definition 
> since the definition defines the declaration. Unfortunately this appears 
> to lead to the fact that if the .d file changes then any other .d file 
> which imports the original one ( has a dependency on the original one ) 
> needs to be recompiled.

But it is quite possible to separate D modules into "headers" and 
"implementations", if desired. Phobos does this for the gc, for example.
It can also be done automatically by generating .di files.