Cleanup pymalloc:

* Rename _PyObject_Alloc() to pymalloc_alloc()
* Rename _PyObject_FreeImpl() to pymalloc_free()
* Rename _PyObject_Realloc() to pymalloc_realloc()
* pymalloc_alloc() and pymalloc_realloc() don't fallback on the raw
  allocator anymore, it now must be done by the caller
* Add "success" and "failed" labels to pymalloc_alloc() and
  pymalloc_free()
* pymalloc_alloc() and pymalloc_free() don't update
  num_allocated_blocks anymore: it should be done in the caller
* _PyObject_Calloc() is now responsible to fill the memory block
  allocated by pymalloc with zeros
* Simplify pymalloc_alloc() prototype
* _PyObject_Realloc() now calls _PyObject_Malloc() rather than
  calling directly pymalloc_alloc()

_PyMem_DebugRawAlloc() and _PyMem_DebugRawRealloc():

* document the layout of a memory block
* don't increase the serial number if the allocation failed
* check for integer overflow before computing the total size
* add a 'data' variable to make the code easiler to follow

test_setallocators() of _testcapimodule.c now test also the context.