Restrict fold exps

[amaanq]

Closes #201
Closes #212

Does fail with this input though, but maybe the tradeoff is worth it

template <typename... Args> void negateValues(Args... args) {
  ((std::cout << !args << " "), ...);
}

[aryx]

can you rebase @amaanq ? Seems like an important regression fix.

[jdrouhard]

The case it breaks is more common in modern c++ so we need to resolve that conflict first.