Fast versions of C functions

We can avoid calling C functions in some common cases as an optimisation.

e.g. Py_IncRef_Fast can just increment the refcount.

e.g. Py_DecRef_Fast can just decrement the refcount provided it is >1.

e.g. PyObject_IsInstance_Fast can check if the type is exactly the same for a fast positive. Can use the type flags to get some fast negatives when one side is a builtin type.

The issue names Py_IncRef_Fast, Py_DecRef_Fast, and PyObject_IsInstance_Fast but gives no file or test paths. Start by locating the existing wrappers or call sites for these C functions, then compare their behavior with the proposed common-case fast paths. Done means the optimized paths preserve existing semantics and handle the stated fast cases.

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