Add Bool.then

This method is useful if you want to create an Option based on the value
of a Boolean, i.e. something like this:

    if foo {
      Option.Some(thing)
    } else {
      Option.None
    }

Using Bool.then, we can reduce this to the following:

    foo.then fn { thing }

Changelog: added

std/src/std/bool.inko

@@ -27,6 +27,17 @@ class builtin Bool {
   fn pub false? -> Bool {
     self == false
   }
+
+  # Calls the supplied closure and wraps its return value in a `Some` if `self`
+  # is `true`, otherwise returns a `None`.
+  #
+  # # Examples
+  #
+  #     true.then fn { 10 }  # => Option.Some(10)
+  #     false.then fn { 10 } # => Option.None
+  fn pub then[T](func: fn -> T) -> Option[T] {
+    if self { Option.Some(func.call) } else { Option.None }
+  }
 }
 
 impl ToInt for Bool {

std/test/std/test_bool.inko

@@ -54,4 +54,9 @@ fn pub tests(t: mut Tests) {
     t.equal(fmt(true), 'true')
     t.equal(fmt(false), 'false')
   }
+
+  t.test('Bool.then') fn (t) {
+    t.equal(true.then fn { 10 }, Option.Some(10))
+    t.equal(false.then fn { 10 }, Option.None)
+  }
 }