Adds #[cbor(skip_if = "<path>")].

This attribute can be attached to fields in structs and enums and prevents those fields from being encoded if the predicate function denoted by path returns true. This function must satisfy the following signature:

fn pred<T>(_: &T) -> bool {
    todo!()
}

Field types must implement Default, and when decoding, the fields are initialised with Default::default() if no value is present.

Please note that skip_if is mutually exclusive with nil, is_nil, and has_nil.

Addresses #43.