marshal currently has a simple, coherent, high-level interface: dump and load serializations.

_fix_co_filename sounds a bit hackish and ad hoc. Martin's suggestion looks to me like a better way to publicly expose filename setting.
Making a function public indefinitely freezes the api and behavior, so we need to consider if we might want to change it.

If Martin is correct as to the definition of _w_long, its a trivial one line convenience function that is not particularly specific to marshal and that could have been inlined in the code (and for all I know, is, when C compiled). Its existence in marshal really seems to me like an implementation detail that should remain private.