Look at using from_bytes

[dopplershift]

Python 3.2 added int.from_bytes, which allows easily generating an integer value from the bytes--and we can use it now that we're only Python >= 3.6.

I doubt we should replace all usages of struct with this, but there are some places in the binary I/O code where we do some jumping through struct hoops just to form a couple of integers. This would certainly be clearer--we'd also want to do a bit of benchmarking to see if there's any performance ramifications.