Improve the indexing in the Array class to be more like native python
list/array types. No need to use `t[c_int(0)]`, instead `t[0]` is
sufficient, for instance.

Add tests for the above. Relies on a new bpf_module function for
exposing the max_entries property of a table in order to range-check the
indices.

In one case, array was using a struct key type.
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>