Improve struct compatibility with CPython

[dhalbert]

• Add 'x' format character to struct format strings. This designates a padding byte that is 0 when packing and is skipped when unpacking.
• Follow CPython's lead on checking buffer length. struct.unpack() requires the buffer size to match exactly the size required by the format string. struct.unpack_from() requires the buffer size to be at least the size required by the format string (the buffer could be longer).
• struct.unpack_from(fmt, data, offset=0) now allows offset as a keyword arg. (fixes ustruct.unpack_from(fmt, data, offset=0) doesn't allow offset to be a keyword #984)

I wanted to also add range checking for the arguments for packing (#1451), but that's more difficult, due to how integers are packed internally. Anyway, this gets us closer to compatibility. We had issues with some drivers running on Linux systems with blinka because of the casual checking of struct up to now.

Note that we split off from MicroPython when we copied struct from modstruct.c to shared-bindings / shared-module. I made corresponding changes in modstruct.c but they aren't used in CircuitPython.

Related Micropython issues:
micropython#3736
micropython#1208

[ladyada]

i like this idea - i could use it a lot in libraries! 'x' as well as strictness

[dhalbert]

i like this idea - i could use it a lot in libraries! 'x' as well as strictness

I was doing a lot of

_, foo, bar, _ = struct.unpack(...)

and got tired of that. It's in regular Python, so we may as well have it. Didn't take a lot to add.

[dhalbert]

Travis is 😃 finally. I always forget to do a make translate after a merge.

[tannewt]

Great! Thank you for more compatibility!