syntax: add `ast::ItemKind::MacroDef`, simplify hygiene info
This PR
- adds a new variant `MacroDef` to `ast::ItemKind` for `macro_rules!` and eventually `macro` items,
- [breaking-change] forbids macro defs without a name (`macro_rules! { () => {} }` compiles today),
- removes `ast::MacroDef`, and
- no longer uses `Mark` and `Invocation` to identify and characterize macro definitions.
- We used to apply (at least) two `Mark`s to an expanded identifier's `SyntaxContext` -- the definition mark(s) and the expansion mark(s). We now only apply the latter.
r? @nrc