Release 0.4.0

[DanieleDiBenedetto]

Changelog:

Imported new curves:

• Import ed25519 #139 - Import ed25519 curve
• Import secp256k1 #122 - Import secp256k1 curve

Support for nonnative arithmetic in circuits:

• Bitcoin circuit support #120 - Imported NonNativeFieldGadget from https://github.com/arkworks-rs/nonnative and built a NonNativeGroupAffineGadget on top of that;
• Non-native field gadgets corrected #143 - Review and partial refactoring of the nonnative module;
• Optimize non native field mul for pseudo mersenne primes #164 - Optimization of nonnative field gadget arithmetic for pseudo-mersenne prime fields

Circuit optimizations:

• Density Optimized Tweedle Fr Poseidon Hash #140 - Poseidon Hash Gadget implementation for Tweedle Fr minimizing densities
• Endo #128 - Endomorphism-based scalar multiplication primitive and gadget
• Imported and generalized Hopwood's vbSM (Introduced Hopwood optimized vbSM gadget #126) and fbSM (Hopwood Optimized Fixed-Base Scalar Multiplication #129) from Faster variable-base scalar multiplication in zk-SNARK circuits zcash/zcash#3924

New additions:

• Bitcoin circuit support #120 - Import sha256 gadget from https://github.com/zkcrypto/bellman/blob/main/src/gadgets/sha256.rs and implemented RIPEMD160 gadget
• In-memory "lazy" sparse merkle tree #142 - Implementation of an in-memory Sparse Field-based Merkle Tree