Validate and fix temp releasing (GH-3708) (GH-3717)

* Validate and fix temp releasing (GH-3708)
Backports 92147baf11071352ffbfa475d0d21e091753e628.

    * 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.