Replace secp_utils usage of `big.Int` with uint256 library.

[rodrigo-pino]

In ./pkg/hintrunner/utils/secp_utils.go there are utilities for elliptic curves. They used big.Int unnecessarily because all values are smaller than 2**256 - 1.

The task is to replace all use of big.Int with uint256 which is waaaaaaay faster.