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: https://github.com/rust-lang/libs-team/issues/196