Switch `value_t` to be `size_t`.

This allows `value_t` to be 64-bit on 64-bit architectures where `unsigned long` is still 32-bit.

The main reason I want to do this is to guarantee the ability to store pointers to other objects inside a hat-trie.

I realize this violates some of the cache locality that hat-tries provide, but I need store store a bundle of records at the end of the hat-trie and this seems like the most efficient means to achieve it. I can cast my pointer to a `value_t` and still be safe knowing I have an appropriately sized integer on the other size.

On Linux, this should make no difference at all since sizeof(unsigned long) == sizeof(size_t).