It is true that tuples and lists are very similar, and when e.g. a
python function accepts tuple as input, and then uses only indexes
access, it can be substituted with list.

However some functions do `isinstance(..., tuple)` etc, and e.g. for a
go program which wants to produce pickle for such python programs there
is currently no way to generate pickle with tuple opcodes.

So to solve this teach ogórek about tuples:

- introduce simple Tuple type which has []interface{} as underlying
- when decoding tuple opcodes produce this Tuple instead of
  []interface{} used previously.
- when encoding encode Tuple with tuple opcodes.

Otherwise Tuple can be seen and behaves like a regular go []interface{}
slice. In fact the only difference is that runtime type of Tuple is
different from runtime type of []interface{} but otherwise both values
are the same and can be casted to each other if/when needed freely.

NOTE opReduce decoder is adjusted to always require tuple, not list,
because with e.g. cPickle:

    "c__main__\nf\n]R." -> TypeError('argument list must be a tuple', ...)
    "c__main__\nf\n)R." -> ok

( the first one uses empty list - "]", and the second one empty tuple - ")" )

So it is ok and compatible to always require args to be tuple for
reduce.