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
I already answered in a followup source comment to the other pull request.
It's sarcasm about Sage development. Do you want me to remove it?

On Tue, 9 Dec 2014 22:38 Aaron Meurer <notifications@github.com> wrote:

> In sympy/external/tests/test_sage.py
> <https://github.com/sympy/sympy/pull/8592#discussion-diff-21564654>:
>
> > @@ -184,4 +218,11 @@ def test_issue_4023():
> >      sage: test_functions()
> >      sage: test_issue_4023()
> >
> > +Hell freezes over before Sage has a symbolic Lucas function::
>
> I think I asked this before but it must have been lost. Why is this?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/sympy/sympy/pull/8592/files#r21564654>.
>