libsyntax: Remove the use foo = bar syntax from the language in favor

of `use bar as foo`.

Change all uses of `use foo = bar` to `use bar as foo`.

Implements RFC #47.

Closes #16461.

[breaking-change]

src/liballoc/lib.rs

@@ -86,7 +86,7 @@ extern crate libc;
 
 #[deprecated = "use boxed instead"]
 #[cfg(not(test))]
-pub use owned = boxed;
+pub use boxed as owned;
 
 // Heaps provided for low-level allocation strategies