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/test/compile-fail/resolve-conflict-import-vs-extern-crate.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use std = std::slice; //~ ERROR import conflicts with imported crate
+use std::slice as std; //~ ERROR import conflicts with imported crate
 
 fn main() {
 }

src/test/run-pass/extern-mod-ordering-exe.rs

@@ -12,7 +12,7 @@
 
 extern crate extern_mod_ordering_lib;
 
-use the_lib = extern_mod_ordering_lib::extern_mod_ordering_lib;
+use extern_mod_ordering_lib::extern_mod_ordering_lib as the_lib;
 
 pub fn main() {
     the_lib::f();