merge_iter: don't compare generators with < via heapq

merge_iter: don't compare generators with < via heapq

Previously we were implicitly comparing (entry, reader_generator) with
"<" in the merge_iter heapq.  In python 2, the generators would just
compare as pointers, which was fine, because we only cared about the
ordering of the initial entries.

However, in Python 3 comparing generators provokes a TypeError, so
create a trivial helper class that can carry the two items, but only
compare the entries via the < operator.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>