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/collections/hash/set.rs

@@ -21,7 +21,9 @@ use hash::{Hash, Hasher, RandomSipHasher};
 use iter::{Iterator, IteratorExt, IteratorCloneExt, FromIterator, Map, Chain, Extend};
 use ops::{BitOr, BitAnd, BitXor, Sub};
 use option::Option::{Some, None, self};
-use result::Result::{Ok, Err};
+
+// NOTE: for old macros; remove after the next snapshot
+#[cfg(stage0)] use result::Result::{Ok, Err};
 
 use super::map::{self, HashMap, Keys, INITIAL_CAPACITY};