Rollup merge of #57579 - stjepang:once-with, r=SimonSapin

Add core::iter::once_with()

Functions `iter::once()` and `iter::repeat()` construct iterators from values. The latter has the lazy variant `iter::repeat_with()`, but the former doesn't. This PR therefore adds `iter::once_with()`.

Another way to think of `iter::once_with()` is that it's a function that converts `FnOnce() -> T` into `Iterator<Item = T>`.

If this seems like a reasonable addition, I'll open a tracking issue and update the `#[feature(...)]` attributes.

src/libcore/lib.rs

@@ -79,6 +79,7 @@
 #![feature(extern_types)]
 #![feature(fundamental)]
 #![feature(intrinsics)]
+#![feature(iter_once_with)]
 #![feature(lang_items)]
 #![feature(link_llvm_intrinsics)]
 #![feature(never_type)]