`PyCode_GetCode` could be faster

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This potentially affects tools like coverage.py. It also means anything using co_code in CPython frequently is significantly slower. Note that pre-3.11, most code would assume this operation is O(1), so it's probably called more often than we expect.

We should lazily initialise a hidden _co_code field and cache it there. Currently it creates a fresh bytes object on each co_code access. This is at best O(n).