The cframe member was always being initialized from the PyThreadState's root_cframe, which doesn't properly have use_tracing set, so new greenlets didn't get tracing.

Here, we do exactly what CPython's ceval.c does and stack-allocate a new CFrame object in g_initialstub and copy the use_tracing value from the currently executing thread state.

This seems to solve the tracing issue, but further tests are needed; none of the existing tests failed with the broken behaviour. In addition to adding an example from #256, we need to test that the use_tracing flag properly propagates up and down when a profiler is set inside a greenlet, likewise I've only been testing switch, need to test throw, and need to test several variations on calling g.run(): as a method, just a target function, an object assigned to a greenlet, etc.

This proves that propagating the tracer *upward* (to a parent greenlet) in Cpython 3.10 fails. However, 4287a40 did fix propagating it downward (to a new greenlet). Likely, propagating among sibling greenlets is also broken.

Because it's stack based, we need to store it separately.