Attached is an updated patch.  I didn't touch Georg's code short of
tossing in some comments on whether we wanted to sneak around some APIs
for performance or not.  Anything I added was to make all but one test pass.

And that one test is test_str.  There is now this odd mixture of
test_unicode, test_str, and test_bytes.  But that doesn't really fit
anymore since unicode is now str, so that file should probably get
renamed.  And str8 is becoming more like bytes.  That means that
string_tests is really just tests for what is now str and we need a
bytes_tests thing for str8 and bytes.

Assigning back to Guido since he seems to be assigned everything that is
holding up 3.0a2.  =)  If I am lucky I will be able to help with this
testing reorg, but that is iffy.