Merge pull request #4739 from royAmmerschuber/feature/refactor-tree-visitor

Move `TreeVisitor` into its own file & make it generic.

Author: RalfJung

src/borrow_tracker/tree_borrows/mod.rs

@@ -13,6 +13,7 @@ pub mod diagnostics;
 mod foreign_access_skipping;
 mod perms;
 mod tree;
+mod tree_visitor;
 mod unimap;
 mod wildcard;