IF: Clean up of bls_private_key, bls_public_key and bls_signature, …

[systemzax]

…removed string prefix, added base58 to_string + parsing code for bls_private_key + unit test, removed old unit test

[systemzax]

Added checksum + wif import / export based on fc::crypto::private_key

[greg7mdp]

Why do we recompute the secret key every time we sign?

   bls_signature bls_private_key::sign( const vector<uint8_t>& message ) const
   {
      std::array<uint64_t, 4> sk = bls12_381::secret_key(_seed);
      bls12_381::g2 sig = bls12_381::sign(sk, message);
      return bls_signature(sig);
   }

[systemzax]

Why do we recompute the secret key every time we sign?

   bls_signature bls_private_key::sign( const vector<uint8_t>& message ) const
   {
      std::array<uint64_t, 4> sk = bls12_381::secret_key(_seed);
      bls12_381::g2 sig = bls12_381::sign(sk, message);
      return bls_signature(sig);
   }

I changed the storage type from vector<uint8_t> to array<uint64_t, 4>, which allows us to avoid recomputing the key every time we sign

[greg7mdp]

@heifner can we merge this one?

[greg7mdp]

@systemzax please push the merge button!