guarantee to keep valid pointers in its slots.

tests:  Moved ExtensionSaver from test_copy_reg into pickletester, and
use it both places.  Once extension codes get assigned, it won't be
safe to overwrite them willy nilly in test suites, and ExtensionSaver
does a thorough job of undoing any possible damage.

Beefed up the EXT[124] tests a bit, to check the smallest and largest
codes in each opcode's range too.

imports e.g. test_support must do so using an absolute package name
such as "import test.test_support" or "from test import test_support".

This also updates the README in Lib/test, and gets rid of the
duplicate data dirctory in Lib/test/data (replaced by
Lib/email/test/data).

Now Tim and Jack can have at it. :)

allows using the tests with unittest.py as a script.  The tests will
still run when run as a script themselves.