Enhancement: make `unpack` return where it stopped reading

[EnTerr]

The code i am trying to port and uses struct needs this feature of Ierusalimschy's struct.unpack:

... After the read values, this function also returns the index in s where it stopped reading, which is also where you should start to read the rest of the string.

So i added it to my copy of lua-struct, as easy as adding a line at the end:

 table.insert(vars, iterator) --<-- one line

 return unpack(vars)