Correct const keyword position in Impl rule

Author: Veykril

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "ungrammar"
 description = "A DSL for describing concrete syntax trees"
-version = "1.9.2"
+version = "1.9.3"
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/matklad/ungrammar"
 authors = ["Aleksey Kladov <aleksey.kladov@gmail.com>"]

rust.ungram

@@ -251,7 +251,7 @@ AssocItem =
 Impl =
   Attr* Visibility?
   'default'? 'unsafe'?
-  'impl' 'const'? GenericParamList? ('!'? trait:Type 'for')? self_ty:Type WhereClause?
+  'impl' GenericParamList? ('const'? '!'? trait:Type 'for')? self_ty:Type WhereClause?
   AssocItemList
 
 ExternBlock =