Remove unnecessary scope for "no strict 'refs'"
Be explicit that mk_translucent() returns the newly minted $accessor method

This is required for the installation of derived class override accessor
methods to work as written.
Make mk_translucent() croak() if wrongly called as an object method

(Idea stolen from comparison with Class::Data::Inheritable.)
Factor out the guts of mk_translucent() in preparation for future expansion

Rename a few variables while we're at it, update the documentation to
match, and regenerate README, which has drifted out of sync with the POD.