Hardcode the string unpack function

[PoroCoco]

The string.unpack function from the string library was only made available since Lua 5.3 and some versions of opencomputer still uses Lua 5.2 by default (eg OC version in FTB Beyond).
This can most of the time be fixed by doing a shift right click while holding the CPU to switch between the 2 versions but it would nonetheless be more pratical not to have to do that.