Move the gap buffer infrasctructure to the bottom of the file
CHUNKSIZE is specific to the gap buffer
changes go through the Buffer object
fix set_mark argument handling

or doesn't work like I think when I'm or'ing 0
fix buf __getitem__

once again, or, plus I forgot negative numbers last time
move cache to Buffer object
fixup cache
make debug messages less verbos

kind of a them, huh?
shouldn't ever happen, but don't faceplant on an empty buffer
with no argument, move to the actual end of the buffer
windows can share abstract buffers, yay

(so C-x 2 on an editor window will do the expected)