[collect] Add method to get i-th element of Collections

This change adds a new method `Collections2.getElement(Collection, int)`, which
returns the i-th element of the collection.

Unlike existing methods via Java `Stream` or `Iterable`, the method in this
change supports fast access for `ImmutableCollection`s backed by an internal
array in `O(1)` instead of `O(n)` with the existing approach.

Users can use this method to, e.g., retrieve a random element from
`ImmutableSet` (e.g., to select a random node to connect to from a pool of
nodes in a distributed system).

Author: Yannic

android/guava/src/com/google/common/collect/Iterables.java

@@ -777,7 +777,9 @@ public Iterator<T> iterator() {
     checkNotNull(iterable);
     return (iterable instanceof List)
         ? ((List<T>) iterable).get(position)
-        : Iterators.get(iterable.iterator(), position);
+        : ((iterable instanceof ImmutableCollection)
+            ? ((ImmutableCollection<T>)iterable).asList().get(position)
+            : Iterators.get(iterable.iterator(), position));
   }
 
   /**

guava/src/com/google/common/collect/Iterables.java

@@ -767,7 +767,9 @@ public Spliterator<T> spliterator() {
     checkNotNull(iterable);
     return (iterable instanceof List)
         ? ((List<T>) iterable).get(position)
-        : Iterators.get(iterable.iterator(), position);
+        : ((iterable instanceof ImmutableCollection)
+            ? ((ImmutableCollection<T>)iterable).asList().get(position)
+            : Iterators.get(iterable.iterator(), position));
   }
 
   /**