fix/update-zkinterface

[tronch0]

Update Zkinterface implementation to match the new ProofSystem trait definition.

TODO:

• Review entire code before.
• Add tests (to zkinterface file, see example g16.rs)
• Get rid of compiler warning?

`warning: unused variable: `vk_path`
  --> zokrates_core/src/proof_system/zkinterface.rs:38:67
   |
38 |     fn setup(&self, program: ir::Prog<FieldPrime>, pk_path: &str, vk_path: &str) {
   |                                                                   ^^^^^^^ help: consider prefixing with an underscore: `_vk_path`
   |
   = note: #[warn(unused_variables)] on by default

warning: unused variable: `pk_path`
  --> zokrates_core/src/proof_system/zkinterface.rs:60:9
   |
60 |         pk_path: &str,
   |         ^^^^^^^ help: consider prefixing with an underscore: `_pk_path`

`