scylla-gdb.py: collection_element: add circular_buffer support
scylla-gdb.py: collection_element: add circular_buffer support

Also add a __getitem__() to circular_buffer and mask indexes so they are
mapped to [`_impl.begin`, `_impl.end`).

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <20200803053646.14689-1-a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

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