Simplify and speed up EUI.ei
Make OUI.__str__ twice as fast as before
Make EUI.ipv6_link_local 5 times faster

Also, properly pass the version as kwarg, not as arg.
Make EUI.eui64 10 times faster
Fix docstring typos
Make IAB.__str__ twice as fast
Avoid unneeded split()+join() when parsing oui.txt/iab.txt
strict is a kwarg, improve subclassability by not hard coding IAB.
string.split('x') always works, even if 'x' not in string.
Increase statement coverage of module rfc1924.
Add unit tests that use IPSet with integers
Add unit tests to improve statement coverage