Validate and fix temp releasing (GH-3708)

* Fix a temp leak in the type init code.
* Fix temp leaks in fused types initialisation code.
* Correctly release the buffer index temps allocated for index calculations.
* Make tests fails hard if a temp variable is not released at the end of a generated function.
* Fix temp leak in switch statement code.
* Make end-to-end tests fail on refnanny output.
* Fix result temp leak in PyTypeTestNode.
* Fix result temp leak in external type/function import code and enable the refnanny check for them.
* Fix temp leak when try-return-finally is used in generators.
* Make it explicit when an allocated temp is not meant to be reused.
* Fix temp leak when assigning to the real/imag attributes of complex numbers.
* Fix temp leak when assigning to a memoryview slice.
* Clean up "num_threads" result temp in parallel section, not only in prange loop.
* Fix temp leak in Pythran buffer setitem code.
* Simplify NumPyMethodCallNode since it does not need the Python function anymore. Previously, it generated code that needlessly looked up the Python function without actually using it.
* Fix temp leak when deleting C++ objects.
* Add a test that actually reusing temps when deleting C++ objects works correctly.