Revert "#TEMP remove sync module to work around LDWRdPtr elaboration bug"

This reverts commit e661b41.

Author: dylanmckay

src/lib.rs

@@ -170,6 +170,7 @@ pub mod borrow;
 
 pub mod any;
 pub mod array;
+pub mod sync;
 pub mod cell;
 pub mod char;
 pub mod panicking;