SI-9347 Efficient head/tail, if possible, for immutable maps & sets

Most immutable collections, including sets and maps, have a better-than-O(n) method for removing an element.  In those cases, tail and possibly head were overridden so the head/tail pattern can be used with less of a performance penalty.

Speed improvements on head/tail pattern are
(for sets/maps of size 1024, unless otherwise specified):

```
BitSet    190x
HashSet   250x
Set       400x
Set2        9x
Set4       12x
HashMap   430x
ListMap  2500x   // size 128
Map       430x
```

Note: `ListMap` is actually `init`/`last` because it's maintained in reverse order.

Altered ListMap docs to explain that reverse traversal is the fast way to do it.

All tested sets/maps that were already fast are still fast.

Test code is reproduced below, except it does ListSet with head/tail which
doesn't show the improvement:

```scala
object BenchTailSetMap {
  val th = new ichi.bench.Thyme

  val standard = 1 to 1024

  val sets = Map[String, Set[Int]](
    "Set" -> (Set.empty[Int] ++ standard),
    "Set4"-> Set(4, 7, 2, 1),
    "Set2"-> Set(3, 4),
    "HashSet" -> (collection.immutable.HashSet.empty[Int] ++ standard),
    "BitSet" -> (collection.immutable.BitSet.empty ++ standard),
    "SortedSet" -> (collection.immutable.SortedSet.empty[Int] ++ standard),
    "ListSet" -> (collection.immutable.ListSet.empty[Int] ++ standard)
  )

  val pairs = standard.map(i => i -> i.toString)

  // ListMap implementation is HORRIBLE, O(n^3) tail!  Cut down size.
  val maps = Map[String, Map[Int, String]](
    "Map" -> (Map.empty[Int, String] ++ pairs),
    "HashMap" -> (collection.immutable.HashMap.empty[Int, String] ++ pairs),
    "SortedMap" -> (collection.immutable.SortedMap.empty[Int, String] ++ pairs),
    "ListMap" -> (collection.immutable.ListMap.empty[Int, String] ++ pairs.take(128))
  )

  def hts(s: Set[Int]) = {
    var si = s
    var x = 0
    while (si.nonEmpty) {
      x += si.head
      si = si.tail
    }
    x
  }

  def htm(m: Map[Int, String]) = {
    var mi = m
    var x = 0
    while (mi.nonEmpty) {
      x += mi.head._2.length
      mi = mi.tail
    }
    x
  }

  def run() {
    sets.toList.sortBy(_._1).foreach{ case (name, s) => th.pbench(hts(s), s.size, name) }
    maps.toList.sortBy(_._1).foreach{ case (name, m) => th.pbench(htm(m), m.size, name) }
  }
}
```

1 files changed, 21 insertions, 0 deletions

diff --git a/src/library/scala/collection/BitSetLike.scala b/src/library/scala/collection/BitSetLike.scala
index 29369447d1..209b00ebf9 100644
--- a/src/library/scala/collection/BitSetLike.scala
+++ b/src/library/scala/collection/BitSetLike.scala
@@ -204,6 +204,27 @@ trait BitSetLike[+This <: BitSetLike[This] with SortedSet[Int]] extends SortedSe
   def subsetOf(other: BitSet): Boolean =
     (0 until nwords) forall (idx => (this.word(idx) & ~ other.word(idx)) == 0L)
 
+  override def head: Int = {
+    val n = nwords
+    var i = 0
+    while (i < n) {
+      val wi = word(i)
+      if (wi != 0L) return WordLength*i + java.lang.Long.numberOfTrailingZeros(wi)
+      i += 1
+    }
+    throw new NoSuchElementException("Empty BitSet")
+  }
+
+  override def last: Int = {
+    var i = nwords - 1
+    while (i >= 0) {
+      val wi = word(i)
+      if (wi != 0L) return WordLength*i + 63 - java.lang.Long.numberOfLeadingZeros(wi)
+      i += 1
+    }
+    throw new NoSuchElementException("Empty BitSet")
+  }
+
   override def addString(sb: StringBuilder, start: String, sep: String, end: String) = {
     sb append start
     var pre = ""