scylla-gdb.py: collection_element(): add std::tuple support
scylla-gdb.py: collection_element(): add std::tuple support

Accessing the element of a tuple from the gdb command line is a
nightmare, add support to collection_element() retrieving one of its
elements to make this easier.
Message-Id: <0e922e3cf2b13836d0c53171c873cb6e5586acb4@scylladb.com>

scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla