Make `set_public` protected

[federicobarbacovi]

We have a specific interface to deal with public inputs (see special_public_inputs). We should always set public inputs via this interface. To prevent usage of set_public on the single elements (PairingPoints, group points, etc.) we should make the method set_public protected and expose it to the public inputs interface via friendship.

[federicobarbacovi]

While it would be nice to prevent usage of set_public outside of the IO classes defined in special_public_inputs, this would not solve the prevent setting public inputs by hand via a call to set_public_input (method of our builders). Also, making set_public private would require changing some testing code, so it doesn't seem worth it.

I went through the codebase and the we only we set public inputs without using the IO in tests, so that's ok.