Use feature(const_fn_trait_bound) instead of feature(const_fn)

    error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
      --> examples/const_fn_test/tests/test.rs:11:6
       |
    11 | impl<T: IntoIterator> A<T> {
       |      ^
       |
       = note: see issue #57563 <rust-lang/rust#57563> for more information
       = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable

Author: taiki-e

examples/const_fn_test/tests/test.rs

@@ -1,4 +1,4 @@
-#![cfg_attr(nightly, feature(const_fn))]
+#![cfg_attr(nightly, feature(const_fn_trait_bound))]
 #![warn(rust_2018_idioms, single_use_lifetimes)]
 #![allow(dead_code)]