usage of `unsafe` is unsound

[BenjaminBrienen]

See: #187

miri finds issues in the current implementation. This causes downstream crates, including rust-analyzer and wgsl-analyzer to also fail under miri.

[bjorn3]

Duplicate of #163. Also try using miri with tree borrows rather than stacked borrows. Stacked borrows doesn't support the &Header pattern that rowan needs, while tree borrows is just fine with it: rust-lang/unsafe-code-guidelines#256