Enable Shape Arithmetic

[ryanmrichard]

Issue Description

Eventually we're going to have to propagate Shape and Allocator instances through the expression layer. This issue is for Shape.

IMHO propagation is easier if we define arithmetic operations for Shape. For example:

// Let these be the shapes for tensors A, B, and C respectively
Shape a, b, c;

// Now if we want c to be the shape C has after doing C("i,j") = A("i,j") + B("i,j")
// we compute that by:
a.add("i,j", "i,j", c, "i,j", b);

The syntax here mirrors that of the Buffer class.

For implementing the arithmetic it's a matter of working out the dimensions given the input indices and the operation. The trickiest part is going to be accounting for sparsity in the derived class if need be.