add Unpack for unpacking integers.

[Vizonex]

Based off #32 I decided to try writing a better and simpler approach that is based off MulAdd but for implementing rshifting and lshifting based on weather or not big or little endians are to be used. In my case I'm going to write a new socks5 protocol parser in C.

I have 2 more forks that will be merged since it's a new function and Node class but I've ensured that there is at least some tests available for it. I do have some skills I've learned from contributing to aiohttp and some of it's lower level C/Cython related code so I should be able to navigate things if the C code is wrong or you think it could be better.

I'll upload those pull requests shortly...

Edit: here's a code snippet from one of the tests I added if anybody is confused about how this new code method works

const start = b.node('start');
    b.property('i16', 'unpack_value');
    const unpack_error = b.error(0, 'failed to unpack value');
    const invoke = b.invoke(
      b.code.unpack('unpack_value', Endianess.Little),
      start, unpack_error
    );