Add Iterator.select

This method allows one to select specific values from an Iterator. This
requires some casts to work around the issue described in
https://gitlab.com/inko-lang/inko/issues/177. We will solve this as part
of the self-hosting compiler project.

runtime/src/std/iterator.inko

@@ -266,6 +266,44 @@ trait Iterator!(T) {
     False
   }
+  # Returns an `Iterator` that only produces values for which the supplied block
+  # returned `True`.
+  #
+  # # Examples
+  #
+  # Selecting only certain values in an `Iterator`:
+  #
+  #     Array.new(10, 20, 30)
+  #       .iter
+  #       .select do (value) { value > 10 }
+  #       .to_array # => Array.new(20, 30)
+  def select(block: do (T) -> Boolean) -> Iterator!(T) {
+    let mut found: ?T = Nil
+
+    # The casts to ?Object here are needed until
+    # https://gitlab.com/inko-lang/inko/issues/177 has been implemented.
+    Enumerator.new(
+      while: {
+        { next?.and { (found as ?Object).nil? } }.while_true {
+          let value = next
+
+          block.call(value).if_true {
+            found = value
+          }
+        }
+
+        (found as ?Object).not_nil?
+      },
+      yield: {
+        let yield = found!
+
+        found = Nil
+
+        yield
+      }
+    )
+  }
+
   # Transforms the `Iterator` into an `Array`.
   #
   # This method will advance the iterator to the end.

runtime/tests/test/std/test_iterator.inko

@@ -116,6 +116,29 @@ test.group('std::iterator::Iterator.map') do (g) {
   }
 }
+test.group('std::iterator::Iterator.select') do (g) {
+  g.test('Selecting values from an empy Iterator') {
+    let iter = EmptyIterator.new
+    let vals = iter.select do (value) { value > 10 }.to_array
+
+    assert.true(vals.empty?)
+  }
+
+  g.test('Selecting values from an Iterator with values') {
+    let iter = SimpleIterator.new
+    let vals = iter.select do (value) { value > 10 }.to_array
+
+    assert.equal(vals, Array.new(20, 30))
+  }
+
+  g.test('Selecting values from an Iterator when no values match') {
+    let iter = SimpleIterator.new
+    let vals = iter.select do (value) { value > 50 }.to_array
+
+    assert.true(vals.empty?)
+  }
+}
+
 test.group('std::iterator::Iterator.to_array') do (g) {
   g.test('Converting an Iterator to an Array') {
     let iter = SimpleIterator.new