Audit and reduce unstable usage in script

Reasons behind existing unstable features:

alloc / OIBIT : For deque (which was just copied from rust's source)

collections:

 - `[T].tail()`
 - `to_lowercase()`

core:

 - `FnBox`
 - `intrinsics::breakpoint()`

exit_status: for setting exit status

path_ext: for `is_dir` and `exists()` (can be done in other ways)

step_by/step_trait

std_misc:

 - Thunk
 - DefaultState and other hasher stuff

zero_one: `num::One`

components/util/deque/mod.rs

@@ -50,8 +50,8 @@
 
 pub use self::Stolen::{Empty, Abort, Data};
 
-use alloc::arc::Arc;
-use alloc::heap::{allocate, deallocate};
+use std::sync::Arc;
+use std::rt::heap::{allocate, deallocate};
 use std::marker;
 use std::mem::{forget, min_align_of, size_of, transmute};
 use std::ptr;