std.prelude vs core library

[Sean Kelly]

Another small reason is to enforce decoupling between required 
code and the rest of the library. Back when Phobos was all one 
library, half the library was compiled into every program. The 
runtime writes to stderr, the IO package relies on other 
modules...  Kind of like what happens now if you import 
std.stdio. And while this can be accomplished via deliberate 
effort towards decoupling. But that's really hard to accomplish 
in an open source project.  Functionally, think of core as being 
similar to java.lang.