bpo-38631: Replace compiler fatal errors with exceptions

[vstinner]

• Replace Py_FatalError() calls with regular SystemError exceptions.
• compiler_exit_scope() calls _PyErr_WriteUnraisableMsg() to log the
  PySequence_DelItem() failure.
• compiler_unit_check() uses _PyMem_IsPtrFreed()
• compiler_make_closure(): remove "(reftype == FREE)" comment since
  reftype can also be LOCAL or GLOBAL_EXPLICIT.

https://bugs.python.org/issue38631

[vstinner]

cc @isidentical @lysnikolaou @pablogsal

I injected bugs to check if the SystemError error message is formatted properly.

Error 1:

SystemError: PyST_GetScope(name='self') failed: unknown scope in unit update_sources_depends (31747488); symbols: {'self': 10260, 'moddirlist': 10242, 'os': 6160, 'headers': 2066, 'sysconfig': 6160, 'glob': 6160, 'escape': 6160, 'ext': 2066}; locals: {'self': 0}; globals: {'os': 0, 'path': 1, 'join': 2, 'srcdir': 3}

Error 2:

SystemError: compiler_lookup_arg(name='self') with reftype=5 failed in update_sources_depends; freevars of code '<listcomp>': ('self',)

Error 3:

SystemError: compiler stack_effect(opcode=116, arg=0) failed

It seems like if PySequence_DelItem() fails in compiler_exit_scope(), the compiler can crash. But I prefer to log the exception rather than exiting the whole process immediately. As written in the comment, since c_stack is a list, the function "shouldn't fail":

    /* we are deleting from a list so this really shouldn't fail */

[pablogsal]

Please, preserve the comment for the else branch: /* (reftype == FREE) */

[vstinner]

Please, preserve the comment for the else branch: /* (reftype == FREE) */

See the commit message: the comment is wrong. Try to convert the comment into an assertion.