make Mutability class ready to be used

[dkrenn]

Looking at the code in sage/structure/mutability.pyx, one notices some things that should be changed:

Update the methods in Mutability: The corresponding methods in sage/structure/sequence.py seem to work nicely. E.g. there self._is_immutable = 1 (in set_immutable) was changed to self._is_immutable = True or self._is_immutable got a return and a try-except (in is_immutable))

Add a doctest in require_mutable and require_immutable that uses the class Mutability.

Add doctests to set_immutable and to is_immutable that really test the code (and note just test the code in sage/structure/sequence.py.

Add doctests to all functions.

Maybe: combine _require_mutable and _require_mutable_cdef with cpdef.

See also the discussion "Mutability" on sage-devel https://groups.google.com/forum/#!topic/sage-devel/dnXSgh56Boo

Component: misc

Keywords: mutability doctests

Issue created by migration from https://trac.sagemath.org/ticket/15264