ustruct: Pull in ustruct module from the Micropython.org tree

[robert-hh]

This addresses the issue #63, fixed by Damien George @dpgeorge
(PR just in case no one else rang the bell.)

[danicampora]

@robert-hh thanks, will merge!

[oligauc]

@robert-hh Thanks

[robert-hh]

Do you plan to merge #64 and #65 too?
#64 would serve the needs of at least one other person who had troubles with non-transparent sys.stdin().
#65 implements a functionality which is already documented. So it has either to be implemented, or the documentation has to be changed.