Use Miyaguchi-Preneel construct for LongsightL hash function

[HarryR]

def compress_MiyaguchiPreneel(m_0, m_1, C, e, p):
  H_0 = LongsightL(IV, m_0, C, e, p)
  H_1 = LongsightL(H_0, m_1, C, e, p)
  return (H_0 + H_1 + m_0) % p

The following needs to be implemented:

• [x] LongsightL C++ implementation in libsnark
• [x] test_longsightl program
• [x] Solidity implementation of LongsightL
• [x] Generic libsnark Miyaguchi-Preneel construct
• [x] Solidity Miyaguchi-Preneel construct for LongsightL
• [x] Python implementation of Miyaguchi-Preneel construct
• [x] test_longsightl_hash_mp to test longsightl in the construct
• [x] Solidity tests

This is a fair chunk of work, but should solve the problem.