derive(Default) is too aggressive

I don't really understand the details, so will just list sources:

- #122 (comment)
- https://rust-lang.zulipchat.com/#narrow/stream/122657-t-compiler.2Fwg-nll/topic/rustc-and-polonius.20integration/near/127049249

polonius-engine/src/facts.rs

@@ -64,6 +64,10 @@ pub struct AllFacts<Origin: Atom, Loan: Atom, Point: Atom, Variable: Atom, MoveP
     pub path_accessed_at: Vec<(MovePath, Point)>,
 }
 
+// We impl Default manually because the more convenient
+// `#[derive(Default)]` syntax would result in, for example,
+// `rustc::ty::RegionVid`, an `Atom`, also having to impl `Default`,
+// something that does not make sense for that type.
 impl<Origin: Atom, Loan: Atom, Point: Atom, Variable: Atom, MovePath: Atom> Default
     for AllFacts<Origin, Loan, Point, Variable, MovePath>
 {