Optimize multiexp

[osuketh]

The multiexp function is currently based on the bellman lib.

zero-chain/core/sonic/src/utils.rs

Lines 209 to 214 in 10c89dc

	let result = multiexp(
	&pool,
	(Arc::new(exponent), 0),
	FullDensity,
	Arc::new(scalar)
	).wait().unwrap();

We might optimize it more based on zexe's one which is based on rayon and avoiding use of trait objects.

In addition, we can remove the duplicative collect(), the parameters are into iterator from vec before passed the mutiexp function.

zero-chain/core/sonic/src/utils.rs

Line 194 in 10c89dc

.collect::<Vec<_>>();

[osuketh]

• Faster batch forgery identification
• Pippenger's exponentiation algorithm
• Improved Techniques for Fast Exponentiation
• Faster Multi-Exponentiation through Caching: Accelerating (EC)DSA Signature Verification
• Efficient Multi-Exponentiation
• Fast parallel computation of multi-exponentiation for public key cryptosystems
• Pippenger multiscalar multiplication algorithm dalek-cryptography/curve25519-dalek#249