Replace mark_smob static member functions with non-static members

Replace mark_smob static member functions with non-static members

Since the mark functions generally need to access a number of members,
it is quite more convenient if the mark functions are proper non-static
member functions.


Also contains commit:

Don't use ASSERT_LIVE_IS_ALLOWED in simple smobs

When replacing mark_smob with a proper member function, use of
ASSERT_LIVE_IS_ALLOWED would require access to self_scm () or similar,
and simple smobs don't link back to their SCM cell.  This only
concerns Skyline and Context_mod, so the loss is not all that
large.

[dak, 2014-09-27 14:32:01]

Use Super::mark_trampoline for mark function to allow overriding