hygiene: Rewrite `apply_mark_internal` to be more understandable The previous implementation allocated new `SyntaxContext`s in the inverted order, and it was generally very hard to understand why its result matches what the `opaque` and `opaque_and_semitransparent` field docs promise. ```rust /// This context, but with all transparent and semi-transparent expansions filtered away. opaque: SyntaxContext, /// This context, but with all transparent expansions filtered away. opaque_and_semitransparent: SyntaxContext, ``` It also couldn't be easily reused for the case where the context id is pre-reserved like in #129827. The new implementation tries to follow the docs in a more straightforward way. I did the transformation in small steps, so it indeed matches the old implementation, not just the docs. So I suggest reading only the new version.