Add declarations for PyObject_Malloc()

change declaration of GC related type slot functions to use 'PyObject*' instead of 'object' to prevent accidental reference counting

optimise kwargs dict creation away when it's only being passed on and not otherwise modified (ticket 744)

prevent fallback to absolute cimport when relative cimport is not found; generally clean up relative cimport code

move declarations of PyObject and PyTypeObject from ref.pxd to object.pxd and extend PyTypeObject with further struct fields

undo CheckExact broadening for RHS of optimised arithmetic expressions as Python would still evaluate it to get the right subtype as result