Sharing modulo commutativity, associativity, transpose

[fritzo]

The shared_intermediates memoization mechanism #43 already implements sharing modulo permutation of tensor operands and alpha-renaming of the equation. @dgasmith and @jcmgray suggested in #43 (comment) that we could further quotient by tensor axis permutation. Note that this breaks into transposing the output (a bit easier), and transposing all of the inputs (a bit tougher).

Status

• hashing einsum modulo alpha-renaming of equation
• hashing einsum modulo reordering of inputs
• hashing tensordot modulo reordering of inputs
• hashing einsum modulo transpose of output
• hashing tensordot modulo transpose of output
• hashing tensordot modulo transpose of inputs
• hashing einsum modulo transpose of inputs