Trac #22713: Multiple zeta algebra

**Old description:**

There is code in Pynac's `zeta2_eval` allowing `lst` arguments. Let's
check if that can be interfaced to Python. Is there numerics available
in mpmath or arb?

https://en.wikipedia.org/wiki/Multiple_zeta_function

**New description:**

This ticket is now used to implement the algebra of MZV, using
algorithms of Francis Brown.

Sample computation:
{{{
sage: Z = Multizeta
sage: M = matrix(2,2,[Z(2),Z(3),Z(4),Z(5)])
sage: M.det()
-10*ζ(1,6) - 5*ζ(2,5) - ζ(3,4) + ζ(4,3) + ζ(5,2)
}}}

URL: https://trac.sagemath.org/22713
Reported by: rws
Ticket author(s): Frédéric Chapoton
Reviewer(s): Vincent Delecroix

src/sage/modular/all.py

@@ -42,6 +42,8 @@
 from .etaproducts import (EtaGroup, EtaProduct, EtaGroupElement,
                           AllCusps, CuspFamily)
 
+lazy_import('sage.modular.multiple_zeta', ['Multizeta', 'Multizetas'])
+
 from .overconvergent.all import *
 
 from .local_comp.all import *