make CallOption(s) opaque types
make CallOption(s) opaque types

*** This is a breaking API change ***

This ensures that CallOptions remains immutable, which is easier to
reason about.

hodduc/go-capnproto2,zombiezen/go-capnproto,zombiezen/go-capnproto2,zombiezen/go-capnproto2,hodduc/go-capnproto2,zombiezen/go-capnproto