Lack of support for function pointers

[ggrieco-tob]

Run echidna on this:

pragma solidity ^0.7.1;

contract FunctionArg {

    function echidna_test() external pure returns (bool) {
      return true;
    }

    function ptr(function()external f) public {
        f();
    }
}

will result in:

echidna-test: internal error: method type
CallStack (from HasCallStack):
  error, called at src/EVM/Solidity.hs:339:22 in hevm-0.49.0-GY0kNk5Azi3A3vL1VQ1lX4:EVM.Solidity

which is caused by hevm not parsing correctly that ABI feature.

[ggrieco-tob]

This will not be fixed in Echidna, instead we will wait until hevm provides supports

[samalws]

Implemented this change at https://github.com/samalws/echidna/tree/fnPtr, won't do an echidna PR until the relevant hevm commit (ethereum/hevm@24a6777) is put into an hevm release

[arcz]

I've updated hevm here #986