Reduce allocations in groupBy

  - Avoid lambda allocation in getOrElseUpdate call
  - Avoid intermediate Tuple2 instances when iterating through
    the mutable map.
  - Avoid allocating Map1-4 during building the result if
    the size of the grouped exceeds 4. Intead build a HashMap
    directly.

```
./benchdb list --limit 2 && ./benchdb results --run 5 --run 6 --pivot size  && ./benchdb results --run 5 --run 6 --pivot size --metric "·gc.alloc.rate.norm"
┏━━━━┳━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━┓
┃ ID ┃ Timestamp           ┃ Msg              ┃
┣━━━━╋━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━━━┫
┃  6 ┃ 2020-08-24 07:14:36 ┃ PR 8948          ┃
┃  5 ┃ 2020-08-24 07:06:47 ┃ PR 8948 Baseline ┃
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2 test runs found (limit reached).
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━┳━━━━━┳━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━┓
┃                                                (size) ┃                ┃                ┃      ┃     ┃        128         ┃         512         ┃         2048          ┃         8192          ┃       ┃
┃ Benchmark                                             ┃ (hashCodeCost) ┃ (maxNumGroups) ┃ Mode ┃ Cnt ┃    Score ┃   Error ┃     Score ┃   Error ┃      Score ┃    Error ┃      Score ┃    Error ┃ Units ┃
┣━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━╋━━━━━╋━━━━━━━━━━╋━━━━━━━━━╋━━━━━━━━━━━╋━━━━━━━━━╋━━━━━━━━━━━━╋━━━━━━━━━━╋━━━━━━━━━━━━╋━━━━━━━━━━╋━━━━━━━┫
┃ #5:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 8631.963 ┃ 116.790 ┃ 30969.939 ┃ 456.711 ┃ 119820.013 ┃ 1741.671 ┃ 480813.306 ┃ 7268.723 ┃ ns/op ┃
┃ #6:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 8287.717 ┃ 135.038 ┃ 30672.551 ┃ 540.409 ┃ 119356.530 ┃ 1592.624 ┃ 477570.346 ┃ 6365.010 ┃ ns/op ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━┻━━━━━┻━━━━━━━━━━┻━━━━━━━━━┻━━━━━━━━━━━┻━━━━━━━━━┻━━━━━━━━━━━━┻━━━━━━━━━━┻━━━━━━━━━━━━┻━━━━━━━━━━┻━━━━━━━┛
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━┳━━━━━┳━━━━━━━━━━┳━━━━━━━┳━━━━━━━━━━━┳━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━┳━━━━━━━┓
┃                                                (size) ┃                ┃                ┃      ┃     ┃       128        ┃        512        ┃        2048        ┃        8192        ┃       ┃
┃ Benchmark [·gc.alloc.rate.norm]                       ┃ (hashCodeCost) ┃ (maxNumGroups) ┃ Mode ┃ Cnt ┃    Score ┃ Error ┃     Score ┃ Error ┃     Score ┃  Error ┃      Score ┃ Error ┃ Units ┃
┣━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━━━━━╋━━━━━━╋━━━━━╋━━━━━━━━━━╋━━━━━━━╋━━━━━━━━━━━╋━━━━━━━╋━━━━━━━━━━━╋━━━━━━━━╋━━━━━━━━━━━━╋━━━━━━━╋━━━━━━━┫
┃ #5:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 2192.014 ┃ 0.022 ┃ 13712.051 ┃ 0.076 ┃ 50949.533 ┃ 56.381 ┃ 198544.803 ┃ 1.213 ┃ B/op  ┃
┃ #6:scala.collection.GroupByBenchmark.buildArrayBuffer ┃             32 ┃              8 ┃ avgt ┃  30 ┃ 1896.014 ┃ 0.021 ┃  5224.051 ┃ 0.077 ┃ 17768.204 ┃  0.305 ┃  67176.786 ┃ 1.180 ┃ B/op  ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━┻━━━━━━┻━━━━━┻━━━━━━━━━━┻━━━━━━━┻━━━━━━━━━━━┻━━━━━━━┻━━━━━━━━━━━┻━━━━━━━━┻━━━━━━━━━━━━┻━━━━━━━┻━━━━━━━┛
```

Author: retronym

src/library/scala/collection/TraversableLike.scala

@@ -451,17 +451,23 @@ trait TraversableLike[+A, +Repr] extends Any
   }
 
   def groupBy[K](f: A => K): immutable.Map[K, Repr] = {
-    val m = mutable.Map.empty[K, Builder[A, Repr]]
-    for (elem <- this) {
-      val key = f(elem)
-      val bldr = m.getOrElseUpdate(key, newBuilder)
+    object m extends mutable.HashMap[K, Builder[A, Repr]] {
+      override def entriesIterator: Iterator[mutable.DefaultEntry[K, Builder[A, Repr]]] =
+        super.entriesIterator
+    }
+    val newBuilderFunction = () => newBuilder
+    for (elem <- this.seq) {
+      val key  = f(elem)
+      val bldr = m.getOrElseUpdate(key, newBuilderFunction())
       bldr += elem
     }
-    val b = immutable.Map.newBuilder[K, Repr]
-    for ((k, v) <- m)
-      b += ((k, v.result))
-
-    b.result
+    val it = m.entriesIterator
+    val m1 = if (m.size > 4) immutable.HashMap.newBuilder[K, Repr] else immutable.Map.newBuilder[K, Repr]
+    while (it.hasNext) {
+      val entry = it.next()
+      m1.+=((entry.key, entry.value.result()))
+    }
+    m1.result()
   }
 
   def forall(p: A => Boolean): Boolean = {