Add PyFrame_GetVar() and PyFrame_GetVarString() functions to get a
frame variable by its name.
Move PyFrameObject C API tests from test_capi to test_frame.
* `PyFrame_FastToLocalsWithError` and `PyFrame_LocalsToFast` are no longer called during profile and tracing.
(Contributed by Fabio Zadrozny)
* Make accesses to a frame's `f_locals` safe from C code, not relying on calls to `PyFrame_FastToLocals` or `PyFrame_LocalsToFast`.
* Document new `PyFrame_GetLocals` C-API function.