Make From impls of NonZero integer const.

I also changed the feature gate added to `From` impls of Atomic integer to `const_num_from_num` from `const_convert`.

library/core/src/lib.rs

@@ -115,6 +115,7 @@
 #![feature(const_likely)]
 #![feature(const_maybe_uninit_as_ptr)]
 #![feature(const_maybe_uninit_assume_init)]
+#![feature(const_num_from_num)]
 #![feature(const_option)]
 #![feature(const_pin)]
 #![feature(const_replace)]