Proper transcript handling

[davidnevadoc]

After the discussion on how to properly add the Public Inputs to the transcript (#134 ) we found that other parts of the code were also mishandling the transcript. For the addition of custom gate related evaluations:

• plonk/plonk-core/src/proof_system/proof.rs

  Lines 288 to 295 in 377c45f

  	self.evaluations
  	.custom_evals
  	.vals
  	.iter()
  	.for_each(|(label, eval)| {
  	let static_label = Box::leak(label.to_owned().into_boxed_str());
  	transcript.append(static_label.as_bytes(), eval);
  	});

This issue should be addressed together with the new design for custom gates in #128