Add additional documentation in core::io.

Added docs for stdout, stderr, print, and println.
Added note about prelude inclusion.
Fix trailing whitespace
Reexport static trait methods on traits in the same module.
Remove unused variable.
Explain an odd conditional check.
auto merge of #6432 : thomaslee/rust/issue-4202-02, r=catamorphism

My earlier fix for #4202 would not work correctly if the trait being exported was a top-level item relative to the module from which it was being exported. An example that would not work correctly with the original patch:

    // foo.rs

    pub use Foo = self::Bar;

    pub trait Bar {
      pub fn bar() -> Self;
    }

    impl Bar for int {
      pub fn bar() -> int { 42 }
    }

    // bar.rs

    fn main() {
      foo::Foo::bar()
    }

This is now supported.

I believe this change will allow the GenericPath trait to be exported from core::path as Path in such a manner, which should allow #5389 to move forward.
auto merge of #6619 : steveklabnik/rust/stdout_docs, r=thestinger

Added docs for stdout, stderr, print, and println.
Add a better introduction for the io module.

Let's actually give a top-level description of what's in here, eh?
Implement unimplemented const binops
Fix LLVMConst{I,F}Cmp
Test fixes, use LLVMConstFCmp in ConstFCmp
Fix wording per feedback

Thanks @catamorphism!
Removing some warnings from logging.rs

These imports were not being used.
auto merge of #6625 : cmr/rust/constops, r=Aatch
auto merge of #6624 : steveklabnik/rust/io_prelude, r=catamorphism

Let's actually give a top-level description of what's in here, eh?
auto merge of #6627 : steveklabnik/rust/remove_warnings, r=thestinger,catamorphism

These imports were not being used.
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