Storable: throw exception on huge values

Storable: throw exception on huge values

The Storable data format is incapable of representing lengths of 2**31 or
greater; and if you try, you can get segfaults or corrupt data or other fun
and games.

Though it would be undeniably good to fix this properly, this is just a
simple starting point: the limitation is documented, and an exception is
thrown when such data is encountered.

Signed-off-by: Reini Urban <rurban@cpanel.net>
Conflicts:
	dist/Storable/Storable.pm
	dist/Storable/Storable.xs