unpack_from is faster than unpack of slice

[landfillbaby]

tiny bit faster
struct.unpack_from just doesn't read the memory further if input data is too long, so slice stops were unneeded
for non-zero slice starts, struct.unpack_from just adds to the memory offset instead of copying like slices
(was 2 commits, rebased to 1 after changing 3 more files)