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

@@ -696,7 +696,7 @@ impl Unifier {
                 self.set_a_to_b(a, x);
             }
             (TVar { fields: Some(fields), range, is_const_generic: false, .. }, TTuple { ty }) => {
-                let len = ty.len() as i32;
+                let len = i32::try_from(ty.len()).unwrap();
                 for (k, v) in fields {
                     match *k {
                         RecordKey::Int(i) => {

nac3standalone/src/basic_symbol_resolver.rs

@@ -74,7 +74,8 @@ impl SymbolResolver for Resolver {
         if let Some(id) = str_store.get(s) {
             *id
         } else {
-            let id = str_store.len() as i32;
+            let id = i32::try_from(str_store.len())
+                .expect("Symbol resolver string store size exceeds max capacity (i32::MAX)");
             str_store.insert(s.to_string(), id);
             id
         }

nac3standalone/src/main.rs

@@ -247,6 +247,8 @@ fn handle_assignment_pattern(
 }
 
 fn main() {
+    const SIZE_T: u32 = usize::BITS;
+
     let cli = CommandLineArgs::parse();
     let CommandLineArgs {
         file_name,
@@ -287,7 +289,6 @@ fn main() {
         // The default behavior for -O<n> where n>3 defaults to O3 for both Clang and GCC
         _ => OptimizationLevel::Aggressive,
     };
-    const SIZE_T: u32 = 64;
 
     let program = match fs::read_to_string(file_name.clone()) {
         Ok(program) => program,