Support integration with other tree changing libxml2 based libraries

I am working on an integration of `lxml` and `libxmlsec` (the
XML security library) and I have hit an important problem:
`libxmlsec` functions can change the libxml2 document (tree)
and thereby seriously confuse `lxml`.

The major problem is that `libxmlsec` may unlink and release subtrees
leading to a `SIGSEGV` in `lxml` code when it later accesses those subtrees.
Fortunately, `libxmlsec` can be told not to release unlinked
subtrees but leave that to the application. But now, my application
must do that: release the subtree if and only if `lxml` will not do
that at a later time (because it has a reference to some node in the subtree).
Looking at the public `lxml` API, I have not found
such a function. I have come up with the following first version
of an `lxml_safe_release`:

cdef int lxml_safe_release(_Document doc, xmlNode* c_node) except -1:
  # we let `lxml` get rid of the subtree by wrapping *c_node* into a
  #  proxy and then releasing it again.
  if elementFactory(doc, c_node) == NULL: return -1
  return 0

I hope that this will be sufficient to prevent SIGSEGV.
However, I doubt that it is already enough that references into
unlinked subtrees really work correctly. In similar situations,
`lxml` calls `moveNodeToDocument` in order to get namespace references
inside the unlinked subtree self contained. `moveNodeToDocument` is not
public and far to complicated that I would like to include a copy
in my code.

I propose that future `lxml` versions should include a public