Implement semitransparent hygiene

Or macro_rules hygiene, or mixed site hygiene. In other words, hygiene for variables and labels but not items.

The realization that made me implement this was that while "full" hygiene (aka. def site hygiene) is really hard for us to implement, and will likely involve intrusive changes and performance losses, since every `Name` will have to carry hygiene, mixed site hygiene is very local: it applies only to bodies, and we very well can save it in a side map with minor losses.

This fixes one diagnostic in r-a that was about `izip!()` using hygiene (yay!) but it introduces a huge number of others, because of #18262. Up until now this issue wasn't a major problem because it only affected few cases, but with hygiene identifiers referred by macros like that are not resolved at all. The next commit will fix that.

crates/hir-expand/src/name.rs

@@ -18,6 +18,8 @@ use syntax::utils::is_raw_identifier;
 #[derive(Clone, PartialEq, Eq, Hash)]
 pub struct Name {
     symbol: Symbol,
+    // If you are making this carry actual hygiene, beware that the special handling for variables and labels
+    // in bodies can go.
     ctx: (),
 }