Add more allocators for default classes.

1. Add `kNoAlloc` constant to `Class` which can be used to disable
   allocating new instances of classes.
2. Make sure that each class in our API has some kind of allocator, or
   allocation of instances disabled.
3. Add some random API methods which are just useful/fun.
Merge pull request #48 from peelonet/allocators

Add more allocators for default classes.