[WIP] Batch verification for ECDSA

[davidnevadoc]

The purpose of this PR is to explore different methods for efficient batch verification of ECDSA signatures.
Particularly in the secp256k1 curve.

Approach 1

To accomplish this we make 2 changes in this PR:

1. Add mul_batch_ecdsa function for ecc chip
2. Add batch_verify function to the ecdsa chip

Batch size	NºRows
1	143 K
4	568 K
6	851 K
8	1.143 K

(142K rows per signature)

Approach 2

Using ECDSA* signatures. (For this approach to be really usable it will need to be integrated with the challenge interface.)
The prover must provide the correct point R for the signature's r parameter.

Batch size	NºRows
8	659 K
12	926K
(77.2K rows per signature)

[enricobottazzi]

Hi @davidnevadoc, is this PR still being worked on?

I'd love such feature :)

[davidnevadoc]

Hey @enricobottazzi ! I'm not working in this feature anymore. Maybe we could merge this, WDYT @kilic ?
I'm still looking into improving ECDSA verification but through other avenues. Mainly, using GLV multiplication (#70) and fixing the generator (if possible).

[enricobottazzi]

Cool!

A little off topic: do you think it would be worth creating a chip PubKeyToAddress or it's better to do this operation outside of the circuit (which in my case means on a smart contract level)