Propagate Allocator through expression layer

[ryanmrichard]

Issue Description

Similar idea to #89, but this time we're adding Expression::allocator. I'm not 100% sure what the signature for Expression::allocator should be, but my guess is:

Allocator allocator(const_annotation_type out_labels, const_shape_reference out_shape);

i.e., it takes the labels and shape for temporary tensor the expression is being assigned to and returns the allocator (I think the shape is needed to work out tilings, for example).

This requires modifying the body of methods like Add::tensor_ along the lines of:

// Get the labels for the three tensors
const auto a_labels  = exp_a.labels(lhs_labels);
const auto b_labels  = exp_b.labels(lhs_labels);
const auto& c_labels = lhs_labels;

// Get the shapes for the three tensors
auto a_shape = exp_a.shape(a_labels);
auto b_shape = exp_b.shape(b_labels);
Shape c_shape;
a_shape.add(a_labels, c_labels, c_shape, b_labels, b_shape);

// Get the allocators for the three tensors
auto a_alloc = exp_a.allocator(a_labels, a_shape);
auto b_alloc = exp_b.allocator(b_labels, b_shape);
Allocator c_alloc;
a_alloc.add(a_labels, c_labels, c_alloc, b_labels, b_alloc);