Add Object.not_nil?

This method is used to check if an object is _not_ nil, making it the
opposite of Object.nil?. This removes the need for writing
`something.nil?.not`, which reads a bit awkwardly.

runtime/src/std/object.inko

@@ -29,4 +29,16 @@ impl Object {
   def nil? -> Boolean {
     _INKOC.object_equals(self, Nil)
   }
+
+  # Returns `True` if `self` is any object other than `Nil`.
+  #
+  # # Examples
+  #
+  # Checking if an object is not `Nil`:
+  #
+  #     10.not_nil?  # => True
+  #     Nil.not_nil? # => False
+  def not_nil? -> Boolean {
+    _INKOC.object_equals(nil?, False)
+  }
 }

runtime/tests/test/std/test_object.inko

@@ -94,3 +94,11 @@ test.group('std::object::Object.nil?') do (g) {
     assert.false(Object.new.nil?)
   }
 }
+
+test.group('std::object::Object.not_nil?') do (g) {
+  g.test('Checking if an object is Nil') {
+    assert.false(Nil.not_nil?)
+    assert.true(10.not_nil?)
+    assert.true(Object.new.not_nil?)
+  }
+}