perf: fix linearity issue in insertIfNew

src/Lean/Data/HashMap.lean

diff --git a/src/Lean/Data/HashMap.lean b/src/Lean/Data/HashMap.lean
index 279cbf6..dc30823 100644
--- a/src/Lean/Data/HashMap.lean
+++ b/src/Lean/Data/HashMap.lean
@@ -122,7 +122,7 @@
     let ⟨i, h⟩ := mkIdx (hash a) buckets.property
     let bkt    := buckets.val[i]
     if let some b := bkt.find? a then
-      (m, some b)
+      (⟨size, buckets⟩, some b)
     else
       let size'    := size + 1
       let buckets' := buckets.update i (AssocList.cons a b bkt) h