Auto merge of #99969 - calebsander:feature/collect-box-str, r=dtolnay

alloc: implement FromIterator for Box<str>

`Box<[T]>` implements `FromIterator<T>` using `Vec<T>` + `into_boxed_slice()`.
Add analogous `FromIterator` implementations for `Box<str>`
matching the current implementations for `String`.
Remove the `Global` allocator requirement for `FromIterator<Box<str>>` too.

ACP: rust-lang/libs-team#196