Auto merge of #16482 - GnomedDev:boxed-subtree, r=lnicola

Swap Subtree::token_trees from Vec to boxed slice

Performs one of the optimizations suggested in #16325, but a little bit more. Boxed slices guarantee `shrink_to_fit` aswell as saving a pointer width as no capacity has to be stored.

Most of the diff is:
- Changing `vec![]` to `Box::new([])`
- Changing initialize -> fill into fill -> into_boxed_slice
- Working around the lack of an owned iterator or automatic iteration over a `Box<[T]>`

I would like to use my own crate, [small-fixed-array](https://lib.rs/small-fixed-array), although I understand if it isn't mature enough for this. If I'm given the go ahead, I can rework this PR to use it instead.

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