Use $crate and macro reexport to reduce duplicated code

Many of libstd's macros are now re-exported from libcore and libcollections.
Their libstd definitions have moved to a macros_stage0 module and can disappear
after the next snapshot.

Where the two crates had already diverged, I took the libstd versions as
they're generally newer and better-tested. See e.g. d3c831b, which was a fix to
libstd's assert_eq!() that didn't make it into libcore's.

Fixes #16806.

src/libstd/io/buffered.rs

@@ -18,11 +18,14 @@ use iter::ExactSizeIterator;
 use ops::Drop;
 use option::Option;
 use option::Option::{Some, None};
-use result::Result::{Ok, Err};
+use result::Result::Ok;
 use slice::{SliceExt};
 use slice;
 use vec::Vec;
 
+// NOTE: for old macros; remove after the next snapshot
+#[cfg(stage0)] use result::Result::Err;
+
 /// Wraps a Reader and buffers input from it
 ///
 /// It can be excessively inefficient to work directly with a `Reader`. For