Fix removal order for dictionary

Switches to `shift_remove` for now. In the future, we should look into a more efficient implementation. Fixes #1258.

src/eval/dict.rs

@@ -88,7 +88,7 @@ impl Dict {
 
     /// Remove a mapping by `key` and return the value.
     pub fn remove(&mut self, key: &str) -> StrResult<Value> {
-        match Arc::make_mut(&mut self.0).remove(key) {
+        match Arc::make_mut(&mut self.0).shift_remove(key) {
             Some(value) => Ok(value),
             None => Err(missing_key(key)),
         }

tests/typ/compiler/dict.typ

@@ -60,6 +60,12 @@
 #test("c" in dict, false)
 #test(dict, (a: 3, b: 1))
 
+---
+// Test that removal keeps order.
+#let dict = (a: 1, b: 2, c: 3, d: 4)
+#dict.remove("b")
+#test(dict.keys(), ("a", "c", "d"))
+
 ---
 // Error: 24-29 duplicate key: first
 #(first: 1, second: 2, first: 3)