Use Pin for the 'don't move' requirement of ReentrantMutex.

The code in io::stdio before this change misused the ReentrantMutexes,
by calling init() on them and moving them afterwards. Now that
ReentrantMutex requires Pin for init(), this mistake is no longer easy
to make.

library/std/src/lib.rs

@@ -266,6 +266,7 @@
 #![feature(format_args_nl)]
 #![feature(gen_future)]
 #![feature(generator_trait)]
+#![feature(get_mut_unchecked)]
 #![feature(global_asm)]
 #![feature(hashmap_internals)]
 #![feature(int_error_internals)]
@@ -293,6 +294,7 @@
 #![feature(panic_info_message)]
 #![feature(panic_internals)]
 #![feature(panic_unwind)]
+#![feature(pin_static_ref)]
 #![feature(prelude_import)]
 #![feature(ptr_internals)]
 #![feature(raw)]