Add Map.values_mut

This method is used to iterate over mutable references to values in a
Map.

This fixes #544.

Changelog: added

std/src/std/map.inko

@@ -257,7 +257,8 @@ class pub Map[K: Hash + Equal[K], V] {
     iter.map fn (e) { e.key }
   }
 
-  # Returns an `Iter` visiting all the values in this `Map`.
+  # Returns an iterator that yields immutable references to the values in
+  # `self`.
   #
   # # Examples
   #
@@ -556,6 +557,23 @@ impl Map if V: mut {
       case index -> @entries.get_mut(index).value
     }
   }
+
+  # Returns an iterator that yields mutable references to the values in `self`.
+  #
+  # # Examples
+  #
+  # Iterating over the values in a `Map`:
+  #
+  #     let mut map = Map.new
+  #
+  #     map.set('name', 'Alice')
+  #
+  #     map.values_mut.each fn (value) {
+  #       value # => 'Alice'
+  #     }
+  fn pub mut values_mut -> Iter[mut V] {
+    iter_mut.map fn (e) { e.value }
+  }
 }
 
 impl Equal[Map[K, V]] for Map if V: Equal[V] {

std/test/std/test_map.inko

@@ -120,6 +120,16 @@ fn pub tests(t: mut Tests) {
     t.true(values.get(0) == 'Alice' or values.get(0) == 'Bla')
   }
 
+  t.test('Map.values_mut') fn (t) {
+    let map = Map.new
+
+    map.set('numbers', [10])
+
+    map.values_mut.each fn (nums) { nums.push(20) }
+
+    t.equal(map.opt('numbers'), Option.Some(ref [10, 20]))
+  }
+
   t.test('Map.opt') fn (t) {
     let map = Map.new