Don't allow the user to define the same field name in a class more th…

…an once.

lang_tests/instance_fields_overlap2.som

@@ -0,0 +1,12 @@
+"
+VM:
+  status: error
+  stderr:
+    ...instance_fields_overlap2.som', line 11, column 9:
+      | a a |
+    Field 'a' has already been defined in this class.
+"
+
+instance_field_overlap2 = (
+    | a a |
+)

src/lib/compiler/ast_to_instrs.rs

@@ -168,7 +168,21 @@ impl<'a, 'input> Compiler<'a, 'input> {
         };
         for var in ast_inst_vars {
             let vars_len = inst_vars.len();
-            inst_vars.insert(lexer.span_str(*var).to_owned(), vars_len);
+            let n = lexer.span_str(*var).to_owned();
+            match inst_vars.entry(n) {
+                hash_map::Entry::Occupied(_) => {
+                    return Err(vec![(
+                        *var,
+                        format!(
+                            "Field '{}' has already been defined in this class.",
+                            self.lexer.span_str(*var)
+                        ),
+                    )])
+                }
+                hash_map::Entry::Vacant(e) => {
+                    e.insert(vars_len);
+                }
+            }
         }
         self.vars_stack.push(inst_vars);