Unconditionally set a mark function on the node wrapper.  We will later
refactor this to use TypedData_Wrap_Struct and we don't want to
conditionally set mark functions on that

Co-Authored-By: Mike Dalessio <mike.dalessio@gmail.com>

This commit introduces a `Noko_Node_Get_Struct` macro for unwrapping
the underlying data pointer from Nokogiri objects. Then we change all
places that unwrap nodes to use the new macro. We also converted
xmlNode to use `TypedData_Wrap_Struct` so we can provide a compaction
function.

Note that we use DATA_PTR instead of "typed" unwrappers everywhere
because the large number of C structs that inherit from xmlNode would
make the unwrapping code unwieldy and type-check-heavy.

Co-Authored-By: Matt Valentine-House <matt@eightbitraptor.com>
Co-Authored-By: Mike Dalessio <mike.dalessio@gmail.com>

This fixes the case where nodes moves and we need to update the private
references.

Co-Authored-By: Matt Valentine-House <matt@eightbitraptor.com>
Co-Authored-By: Mike Dalessio <mike.dalessio@gmail.com>

Co-Authored-By: Mike Dalessio <mike.dalessio@gmail.com>

We don't have `rb_gc_location` everywhere, so check for it in the
extconf and then conditionally set the compaction callback

Co-Authored-By: Mike Dalessio <mike.dalessio@gmail.com>

for warnings triggered by the compaction test.

- for memory allocated in ary_heap_realloc
- for memory added to the mark stack in stack_chunk_alloc