lua: parenthesized expressions truncate multi-return via new lua-paren AST node +2 tests

lib/lua/parser.sx

@@ -281,7 +281,7 @@
                 ((at-op? "(")
                   (begin
                     (advance-tok!)
-                    (set! base (parse-expr))
+                    (set! base (list (quote lua-paren) (parse-expr)))
                     (consume! "op" ")")))
                 (else (error "lua-parse: expected prefixexp")))
               (define