Use cheaper init in thread_local (#701)

`thread_local!` has special case for `const {}` syntax, and avoids creating an extra lazy init function in such case.

Author: kornelski

src/lib.rs

@@ -150,7 +150,7 @@ mod lock {
     static mut LOCK: *mut Mutex<()> = ptr::null_mut();
     static INIT: Once = Once::new();
     // Whether this thread is the one that holds the lock
-    thread_local!(static LOCK_HELD: Cell<bool> = Cell::new(false));
+    thread_local!(static LOCK_HELD: Cell<bool> = const { Cell::new(false) });
 
     impl Drop for LockGuard {
         fn drop(&mut self) {