Add serde default attribute to compile config to accept partial objects

[dark64]

This does not work currently:

zokrates.compile(source, {
    config: {
      allow_unconstrained_variables: true,
    },
  });

The whole config object must be passed:

zokrates.compile(source, {
    config: {
      allow_unconstrained_variables: true,
      isolate_branches: false,
    },
  });

This PR adds #[serde(default)] to compile config fields so partial configs are accepted.

[Schaeff]

Nice, note that this is also included in #988