Rename create and dispose to alloc and dealloc

As discussed in recent pull requests, this helps to clarify that these functions merely allocate and deallocate memory, and helps clarify the difference between destroy and dealloc.

I left alone `gridfile_create`, which both allocates and initializes.

Note also that there is no `mongo_dealloc` – that's handled by `mongo_destroy`.
Merge pull request #105 from noa--/create_to_alloc

Rename create and dispose to alloc and dealloc