gh-147988: Initialize digits in long_alloc() in debug mode (#147989)

When Python is built in debug mode:

* long_alloc() now initializes digits with a pattern to detect usage of
  uninitialized digits.
* _PyLong_CompactValue() now makes sure that the digit is zero when the
  sign is zero.
* PyLongWriter_Finish() now raises SystemError if it detects uninitialized
  digits

Co-authored-by: Serhiy Storchaka <[email protected]>