Enable Allocator Arithmetic

[ryanmrichard]

Issue Description

See #87. This is more or less the same issue, but now on the Allocator class.

Compared to Shape, how to implement the various operations for Allocator is more unclear. As a first pass I'm fine just setting the resulting Allocator to one of the input Allocator objects. E.g.:

void Allocator::add(const_annotation_reference, const_annotation_reference, Allocator& out,
                    const_annotation_reference, const_allocator_reference rhs) {
out = rhs;
}

This works because both of our Allocator specializations (single element tiles, and one-big tile) are agnostic to the operation. At some point derived classes will have more state (e.g., tiling info for each mode) and the operations will need to be fleshed out more at that point.