Followups: MultiToken refactor

[andrew-fleming]

• Redundant canonicalization -> MultiToken refactor -> replace pk with sk #483 (comment)
• followup: _update re-enters exported balanceOf instead of a raw helper -> MultiToken refactor -> replace pk with sk #483 (comment)
• Redundant as Uint<128> casts in _update -> MultiToken refactor -> replace pk with sk #483 (comment)
• that should mirror the circuit hasher so lets leave a comment to highlight that -> MultiToken refactor -> replace pk with sk #483 (comment)