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.