Add top-level collection-manipulation functions.

CHANGELOG.md

+## 1.8.0
+
+* Add a top-level `mapMap()` function that works like `Iterable.map()` on a
+  `Map`.
+
+* Add a top-level `mergeMaps()` function that creates a new map with the
+  combined contents of two existing maps.
+
+* Add a top-level `groupBy()` function that converts an `Iterable` to a `Map` by
+  grouping its elements using a function.
+
+* Add top-level `minBy()` and `maxBy()` functions that return the minimum and
+  maximum values in an `Iterable`, respectively, ordered by a derived value.
+
+* Add a top-level `transitiveClosure()` function that returns the transitive
+  closure of a directed graph.
+
 ## 1.7.0
 
 * Add a `const UnmodifiableSetView.empty()` constructor.
 
 ## 1.6.0
 
 * Add a `UnionSet` class that provides a view of the union of a set of sets.
 
 * Add a `UnionSetController` class that provides a convenient way to manage the
   contents of a `UnionSet`.
@@ skipping 64 matching lines @@
 
 * Fix all analyzer hints.
 
 ## 0.9.3
 
 * Add a `MapKeySet` class that exposes an unmodifiable `Set` view of a `Map`'s
   keys.
 
 * Add a `MapValueSet` class that takes a function from values to keys and uses
   it to expose a `Set` view of a `Map`'s values.