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

Not at all!  Put it up and see what happens.  Granted, there are only so many of us who feel both qualified to review such tickets and also make time for what (to most people, I guess) seem to be fairly obscure things... but always better to have the code up there.