Completely get rid of StringIO.py and cStringIO.c.
I had to fix a few tests and modules beyond what Christian did, and
invent a few conventions. E.g. in elementtree, I chose to
write/return Unicode strings whe no encoding is given, but bytes when
an explicit encoding is given. Also mimetools was made to always
assume binary files.
Not all code has been fixed yet; this is just a checkpoint...
The C API still has PyDict_HasKey() and _HasKeyString(); not sure
if I want to change those just yet.