First part of making IO up to spec

with S16 | S32/IO synopsis updates.

This set of changes makes the .IO coercer return an IO::Path, rather than an
(unopened) IO::Handle.  Minor roast changes were necessary, but just because
the spec changed.  Hope that any problems in the ecosystem, will become clear
quickly: I don't expect many issues, though, apart from some features now
actually working  :-)