Optimize: change contract function visibility to "external"

[paco0x]

In verifyProof function, it uses yul functions like calldataload & return, which will be interpreted to CALLDATALOAD and RETURN opcodes, and these opcodes only make sense when the calling context is an external call.

Currently, the visibility of the generated contract functions are public and may be used unexpected as an internal call by inherit the generated contract, like:

contract MisusingVerifier is FflonkVerifier {
    function verify(...) {
        verifyProof(...)     // this will be an internal call and unexpected results may occur
    }
}

This PR changes public to external to avoid unexpected usage of the verifier contracts.