Added ECDSA256 lib (#120)

* added ecdsa256 lib

* removed random in test

* review fixes: corrected naming

* removed:
comments with formulas;
redundant check

* updated version and lock file

* updated readme

Author: aritkulova

README.md

@@ -10,7 +10,7 @@
 Solidity modules and utilities that **go far beyond mediocre solidity**.
 
 - Implementation of the [**Contracts Registry**](https://eips.ethereum.org/EIPS/eip-6224) pattern
-- State-of-the-art cryptography primitives (**ECDSA over 384-bit curves**, **RSASSA-PSS**)
+- State-of-the-art cryptography primitives (**ECDSA over 256-bit and 384-bit curves**, **RSASSA-PSS**)
 - Advanced data structures (**Vector**, **DynamicSet**, **PriorityQueue**, **AVLTree**)
 - ZK-friendly [**Sparse Merkle Tree**](https://docs.iden3.io/publications/pdfs/Merkle-Tree.pdf) and [**Incremental Merkle Tree**](https://github.com/runtimeverification/deposit-contract-verification/blob/master/deposit-contract-verification.pdf) implementations
 - Versatile access control smart contracts (**Merkle whitelists**, **RBAC**)