std.prelude vs core library

[Ross Hays]

I was reading about Rust and one thing that caught my attention 
as interesting was the inclusion of std::prelude in the beginning 
of every package. I was curious what the advantage of this were 
versus having things declared in object.d for what seems to be 
the same effect.

Also after looking at the source code of Rust on Github, I don't 
see anything in the runtime that mirrors the D core.* modules. I 
know that it isn't required to be there, but I am just curious 
why D took the approach of having some core modules in the 
runtime? Is it just so Phobos doesn't need ported in order for D 
to be ported to a new system?

Sorry if these questions seem a bit out there, just trying to 
learn some more about programming languages design/implementation.

Thanks,

Ross