feat(public API): expose hashing to curve for BN254 and BLS12-381

[mratsim]

For @Mikerah for differential testing/fuzzing vs https://github.com/hashcloak/bn254-hash-to-curve

AFAIK all crypto libraries use the same internal representation:

• native-endian 64-bit words (or 32-bit words on 32-bit machines for Constantine )
• little-endian ordering of limbs withing a BigInt/Field element
• Montgomery representation with a' = aR (mod p) and R = 2²⁵⁶ (mod p) for BN254 (i.e. the next multiple of the word size)
• (x, y) ordering of Elliptic curve coordinates

Hence you should be able to cast Constantine <-> Arkworks similar to what I do for Halo2curves here with mem::transmute::<MaybeUninit<bn254_snarks_g1_aff>, ark_bn254::G1Affine>

constantine/constantine-rust/constantine-halo2-zal/src/lib.rs

Lines 43 to 55 in 9268502

	fn msm(&self, coeffs: &[bn256::Fr], bases: &[bn256::G1Affine]) -> bn256::G1 {
	assert_eq!(coeffs.len(), bases.len());
	let mut result = MaybeUninit::<bn254_snarks_g1_prj>::uninit();
	unsafe {
	ctt_bn254_snarks_g1_prj_multi_scalar_mul_fr_coefs_vartime_parallel(
	self.0.get_private_context(),
	result.as_mut_ptr(),
	coeffs.as_ptr() as *const bn254_snarks_fr,
	bases.as_ptr() as *const bn254_snarks_g1_aff,
	bases.len(),
	);
	mem::transmute::<MaybeUninit<bn254_snarks_g1_prj>, bn256::G1>(result)
	}