Fixup MacroRules to work with xtask codegen

Author: jonas-schievink

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "ungrammar"
 description = "A DSL for describing concrete syntax trees"
-version = "1.2.0"
+version = "1.2.1"
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/matklad/ungrammar"
 authors = ["Aleksey Kladov <aleksey.kladov@gmail.com>"]

rust.ungram

@@ -101,7 +101,7 @@ Item =
 MacroRules =
   Attr* Visibility?
   'macro_rules' '!' Name
-  '{' MacroArm (';' MacroArm)* ';'? '}'
+  '{' (MacroArm (';' MacroArm)* ';'?)? '}'
 
 MacroArm =
   TokenTree '=>' TokenTree