Tigthen function visibilities/scopes in solidity contracts

[AntoineRondelet]

As of now, many functions are kept public on smart contracts to add them to the ABIs and allow the testing scripts to call them (for instance, see the assemble_nullifier, assemble_hsig etc function on the MixerBase contract).
The ABI should only contain functions exposed to the end user. As such, let's switch the scope of public functions to become (at least) internal, and use "test contracts" to wrap these internal functions with public ones. E.g.

contract MyContract{

function myFunctionToTest() internal {...}

}

and use the following wrapping contract to execute the tests:

contract MyContractTest is MyContract {

function myTestFunction() public {
    return myFunctionToTest();
}

}

Then, we can call MyContractTest from the python testing scripts.

Note: We may also want to move more of that test logic to solidity and expose a single entry point for all tests in the "test contract".

[AntoineRondelet]

(Also related to #141)

[AntoineRondelet]

Tackled as part of #361