Stabilize format_args_capture

Works as expected, and there are widespread reports of success with it,
as well as interest in it.

Author: joshtriplett

alloc/src/lib.rs

@@ -105,7 +105,7 @@
 #![feature(fmt_internals)]
 #![feature(fn_traits)]
 #![feature(inherent_ascii_escape)]
-#![feature(format_args_capture)]
+#![cfg_attr(bootstrap, feature(format_args_capture))]
 #![feature(inplace_iteration)]
 #![feature(iter_advance_by)]
 #![feature(iter_zip)]