new unreachable syntax

* `noreturn` is the primitive type.
 * `unreachable` is a control flow keyword.
 * `@unreachable()` builtin function is deleted.

closes #214

src/ast_render.cpp

@@ -216,6 +216,8 @@ static const char *node_type_str(NodeType node_type) {
             return "Break";
         case NodeTypeContinue:
             return "Continue";
+        case NodeTypeUnreachable:
+            return "Unreachable";
         case NodeTypeAsmExpr:
             return "AsmExpr";
         case NodeTypeFieldAccessExpr:
@@ -890,6 +892,11 @@ static void render_node_extra(AstRender *ar, AstNode *node, bool grouped) {
                 fprintf(ar->f, "continue");
                 break;
             }
+        case NodeTypeUnreachable:
+            {
+                fprintf(ar->f, "unreachable");
+                break;
+            }
         case NodeTypeSliceExpr:
             {
                 render_node_ungrouped(ar, node->data.slice_expr.array_ref_expr);