## Why
Due to the usage of `&'alloc mut T` in `oxc_allocator::Box`, and
`bumpalo::collections::Vec` in `oxc_allocator::Vec`, ast types are
currently invariant over their allocator lifetime `'a`. This prevents
`ouroboros` from generating `borrow_*` on ast type fields, leading to
the unfriendly `with_*` api:
c250b288ef/crates/oxc_parser/examples/multi-thread.rs (L82-L84)
## How
- For `oxc_allocator::Vec`, switch to `allocator_api2::vec::Vec`, which
has a covariant relationship with the allocator lifetime.
- For `oxc_allocator::Box`, use `std::ptr::NonNull` which is
specifically designed to be covariant. I don't use
`allocator_api2::boxed::Box` because it holds the allocator for
dropping, so the size is bigger.
## Downside
Now that `oxc_allocator::Box` uses the unsafe `NonNull`. It has to be a
private field to be safe. This make it impossible to do `Box(....)`
pattern matching.
* move `visit` and `visit_mut` modules to a super module called `visit`
* add `walk_mut` module containing walk functions
* update `enter_node` and `leave_node` events to not pass a reference in the `VisitMut` trait
* add `AstType`, a non-referencing version of `AstKind` to use with `VisitMut` trait
* update the `VisitMut` trait's usages.
relates #709
The allocator and lifetime gets in the way if we want to parse in
parallel but process them in a single thread.
This example uses `ouroboros` to provide a safe API for working with
this unsafe behavior.
