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

Write this as an XFAIL test.