Support for components members within entity definitions

- Compose language now supports defining and initializing components
  within entities.
- This is done with a "components" block, see start.cmp for an example.
- Components get added during entity's constructor
- Properties can be initialized with a named-parameter style syntax when
  declaring the component in the entity.
- set_property messages get sent immediately to the component after
  construction, so properties will be set before the init message is
  sent to the component when the entity has been fully formed.