Fix infinite recursion when walking Abstractions

This caused an infinite recursion when trying to minimise the
non-idempotency when running --verify on

    {
      foo
    # bar
    , baz # qux
    }:
    null

src/Nixfmt/Types.hs

@@ -345,7 +345,7 @@ instance LanguageElement Expression where
           )
     (Assert _ cond _ body) -> [cond, body]
     (If _ expr0 _ expr1 _ expr2) -> [expr0, expr1, expr2]
-    (Abstraction param _ body) -> [Abstraction param (ann TColon) (Term (Token (ann (Identifier "_")))), body]
+    (Abstraction param _ body) -> walkSubprograms param ++ [body]
     (Application g a) -> [g, a]
     (Operation left _ right) -> [left, right]
     (MemberCheck name _ sels) -> name : (sels >>= walkSubprograms)