provide Function._sage_; remove superfluous _sage_ methods; add more of them; fixes #3444

this should enable all bindings between symbolic functions in SymPy and Sage which are possible at the moment and, for identical names, in the future; so it fixes #3444 for the moment

depends on #8585

also fixes http://trac.sagemath.org/ticket/14723 and http://trac.sagemath.org/ticket/14446 and http://trac.sagemath.org/ticket/17493

Yes, totally aware of this.  What's wrong with more, *if* you have the code already?  Obviously if not, not worth it yet.  But I have often been surprised by people coming out of the woodwork - yes years later.  Anyway, probably shouldn't continue this discussion here, or really at all.