[DO NOT MERGE] crypto: torsion clearing for FCMP++

[j-berman]

This introduces 4 new crypto functions used in the FCMP++ integration:

• mul8_is_identity
  • Outputs that fail this check cannot enter the FCMP++ tree.
  • Such outputs are not spendable today.
• clear_torsion
  • Outputs must have torsion cleared before they enter the FCMP++ tree.
• get_valid_torsion_cleared_point
  • "valid" means that the point is able to be inserted to the FCMP++ tree.
  • If false, the point cannot enter the tree.
  • In addition to the point decompression and mul8_is_identity check, we also make sure the point is not equal to identity after clearing torsion.
• rct::verPointsForTorsion
  • After the FCMP++ fork, we use this function to stop allowing outputs that fail get_valid_torsion_cleared_point OR have torsion from entering the chain.