Issue 4400: Rework LY_DECLARE_SMOB_PROC to declare a member function

Issue 4400: Rework LY_DECLARE_SMOB_PROC to declare a member function

This is nicer to work with.  LY_DECLARE_SMOB_PROC now needs to
additionally get the class name for declaring the smob procedure: this
is astonishingly hard to avoid.  Moving the initialization out of
Smob_base<Super>::init into a separate routine smob_proc_init defined by
LY_DECLARE_SMOB_PROC considerably simplifies the initialization.