In its existing usage, `CppStream` was only used in a context where the
C++ `READWRITE` macro was being called with a `CDataStream`. However, in
other contexts the macro can be called with various other types with a
stream-like interface. Since we can't expose C++ templates across the
`cxx` bridge (or FFI generally), we instead turn `CppStream` into an
enum that covers all of the stream-like types we may want to use.
Jack Grigg