Make all collection constants always zero

• Currently, array constants are always zero, but product and summation constants can technically contain values.
• We already assume in the backends and in GCM that collection constants (including products and summations) are zero.
• We should implement two changes:
  • Remove sub-constants in IR product and summation constants (@rarbore2).
  • Update frontend to insert write nodes to update constants as needed (@aaronjc4).

assigned to @rarbore2

I've pushed (I think all) the frontend changes to the zero-collection-constants branch.