added safety assertion to prevent utility code from overwriting that of other entries with the same name

use proper module.m_free() way to run module cleanup code in Py3, way after running atexit functions etc.

moved utility code for registering the module cleanup function into Cython/Utility/ModuleSetupCode.c

fix compiler crash during type inference when calling Python method of C int variable (found in Py3 pyregr test_long.py)

Update docs/src/tutorial/caveats.rst

small grammar error/typo in doc