Inline SyntaxContext in both encoded span representation. The current interned representation for spans does not use the `ctxt_or_zero: u16` field. This PR proposes to use this field to store the `SyntaxContext` of the interned span instead. When `ctxt_or_zero` and the interned span's `ctxt` don't match, the inlined one takes precedence. This allows to implement `Span::ctxt` and `Span::with_ctxt` with much less probability to access the interner. Those functions are used a lot for hygiene, so this may be worth it.