Rename allocator?

There are multiple kinds of allocator:

- SliceAllocator: Write to a fixed size slice, does no allocation.
- VecAllocator: Write to a vector, actually does allocation.
- IoAllocator: Write to a seekable I/O object, doesn't do allocation.

I think I should probably rename the `Allocator` trait to something else, since two of these don't do any allocation. I'm thinking `Store` or `Storer`.

Review the Allocator trait and its SliceAllocator, VecAllocator, and IoAllocator implementations to understand how the name is used. Decide on terminology that accurately covers all three implementations, then update the affected references consistently. Done means the naming is clear and no implementation is incorrectly described as allocating.

Written by the indexing model from the issue text.