core: Refactor VarMap to IndexMap

This is the only Map I can find that preserves insertion order while
also deduplicating elements by key.

nac3core/src/typecheck/typedef/mod.rs

@@ -1,5 +1,5 @@
 use std::cell::RefCell;
-use std::collections::{BTreeMap, HashMap};
+use std::collections::HashMap;
 use std::fmt::Display;
 use std::rc::Rc;
 use std::sync::{Arc, Mutex};