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`
