Test failures from sizeof RefList on win64

[Martin Packman]

The following tests fail on win64, under reporting sizeof RefList by 4 bytes and overestimating the total sizeof MemObjectCollection by 4 bytes:

======================================================================
FAIL: test__sizeof__with_dummy (meliae.tests.test__loader.TestMemObjectCollection)
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Traceback (most recent call last):
  File "meliae\tests\test__loader.py", line 257, in test__sizeof__with_dummy
    has_gc=False)
  File "meliae\tests\test__loader.py", line 211, in assertSizeOf
    self.assertEqual(expected_size, _scanner.size_of(obj))
AssertionError: 8344 != 8336

======================================================================
FAIL: test__sizeof__with_reflists (meliae.tests.test__loader.TestMemObjectCollection)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "meliae\tests\test__loader.py", line 249, in test__sizeof__with_reflists
    has_gc=False)
  File "meliae\tests\test__loader.py", line 211, in assertSizeOf
    self.assertEqual(expected_size, _scanner.size_of(obj))
AssertionError: 8344 != 8336

RefList should use size_t not long for its size, and the estimates in the tests need adapting.