Provide NonZero_c_* integers

I'm pretty sure I am going want this for #73125 and it seems like an
omission that would be in any case good to remedy.

It's a shame we don't have competent token pasting and case mangling
for use in macro_rules!.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

library/std/src/lib.rs

@@ -263,6 +263,7 @@
 #![feature(exact_size_is_empty)]
 #![feature(exhaustive_patterns)]
 #![feature(extend_one)]
+#![feature(extended_key_value_attributes)]
 #![feature(external_doc)]
 #![feature(fn_traits)]
 #![feature(format_args_nl)]