Add a bunch of documentation, and somewhat cleanup, the code in src/backend/.

src/ir/ast.rs

@@ -163,6 +163,15 @@ where
     }
 }
 
+impl From<ValueOrRef> for Expression {
+    fn from(value: ValueOrRef) -> Self {
+        match value {
+            ValueOrRef::Value(loc, val) => Expression::Value(loc, val),
+            ValueOrRef::Ref(loc, var) => Expression::Reference(loc, var),
+        }
+    }
+}
+
 #[derive(Debug)]
 pub enum Value {
     Number(Option<u8>, i64),