Add warning for scope of exec

[Eddy114514]

exec behaves differently in Python 2 and Python 3 inside function scope. In Python 2, a plain exec statement can write back into the enclosing function's local variables. In Python 3, exec() does not reliably write back to function locals unless an explicit locals mapping is provided.

Example:
In Python 2
def f(): b = 42 exec "b = 99" return b
return 99

In Python 3
def f(): b = 42 exec("b = 99") return b
return 42 as exec didn't update local b.

In order to allow exec to modify local b. A safer version will be
def g(): b = 42 ns = {"b": b} exec("b = 99", globals(), ns) return ns["b"]

Thus, in order to warn about such scope issue. I mainly modified cevel.c that

1. Before exec runs, snapshot the current fast locals.
2. After exec, compare fast locals and record which local slots changed.
3. If one of those locals is later read (LOAD_FAST), emit the warning.
4. If it is overwritten or deleted before being read, clear the pending state and do not warn.

[ltratt]

This mostly looks OK at a high-level (except the code around track_locals which needs explaining, because it's not obvious) and I have some low-level comments. I also don't know if CPython prefers explicitly saying if (x == NULL) or the terser (and valid, exactly equivalent) if (x). If the latter, the PR can be simplified further.

[Eddy114514]

@ltratt Thank you for your comments, I have fixed the low levels issues. For the track_loclas, I renamed it track_exec_writeback as it is a variable to controls whether we snapshot and later compare fast locals around a plain exec, so we only record bindings that exec actually wrote back. I also added a short comment explaining that path.

For the case of if(x = null) vs if (x), for what I observed in cevel.c, it tend to use the if(x=null).

[ltratt]

Thanks! Please squash.

[Eddy114514]

@ltratt Sorry I had 2 exam this week and I was preparing for it. I have squashed the commits.