refactor project structure (start of redoing atomic branch without atomics)

core/src/util/borrow.rs

@@ -0,0 +1,35 @@
+use std::ops::{Deref, DerefMut};
+
+pub struct DynBorrower<'a, T: ?Sized> {
+    data: &'a mut T,
+    borrowed: &'a mut bool,
+}
+
+impl<'a, T: ?Sized> DynBorrower<'a, T> {
+    pub fn new(data: &'a mut T, borrowed: &'a mut bool) -> Self {
+        if *borrowed {
+            panic!("tried to mutably borrow the same thing twice");
+        }
+        Self { data, borrowed }
+    }
+}
+
+impl<T: ?Sized> Drop for DynBorrower<'_, T> {
+    fn drop(&mut self) {
+        *self.borrowed = false;
+    }
+}
+
+impl<T: ?Sized> Deref for DynBorrower<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        self.data
+    }
+}
+
+impl<T: ?Sized> DerefMut for DynBorrower<'_, T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.data
+    }
+}