reference counting

I've been thinking long and hard about reference counting and methinks it is time to add it to cowdb. Here's the algo: when removing a symbolic link to an object, the object should be deleted iff

1. the object has an immutable name (typically a unique name),
2. the object is not well known and
3. the object no longer has any backlinks.

OK, it is not full garbage collection. Circular structures will still need to be handled by the application programmer. But still it should be quite helpful and will be particularly useful when we get to COODBMS Folders.

I also expect this to be the final major feature to be added to CowDb. (Hmm. I think I said that before. Hopefully this time it will be true and I can turn my full attention then to implementing COODBMS.)