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