Implement PicklableFunction(interactive_function)

[nthiery]

Extend sage.misc.fpickle.pyx with a PicklableFunction class wrapping
interactively defined (simple) functions to make them picklable:

    sage: f = lambda x: x^2
    sage: loads(dumps(f))
    ------------------------------------------------------------
    Traceback (most recent call last):
      File "<ipython console>", line 1, in <module>
      File "sage_object.pyx", line 792, in sage.structure.sage_object.dumps (sage/structure/sage_object.c:8357)
    PicklingError: Can't pickle <type 'function'>: attribute lookup __builtin__.function failed

    sage: f = PicklableFunction(f)
    sage: f(3)
    9
    sage: f == loads(dumps(f))
    True

CC: @sagetrac-sage-combinat

Component: pickling

Keywords: lambda, interactively defined functions

Issue created by migration from https://trac.sagemath.org/ticket/8561

[nbruin]

comment:2

See #11845 for some code in this direction and for warnings why this should never be allowed in "standard" pickles.