bt2: make `_EventConst` a mapping

bt2: make `_EventConst` a mapping

The `_EventConst` class already implemented the __getitem__() method
to make it easy to access a payload, specific context, common context,
or packet context field (in this order) by name, for example:

    print(event['next_prio'])

If two root fields contain fields which share the same name, one of them
has the priority and the other one is not accessible through this
interface.

This patch makes the `_EventConst` class a full mapping protocol,
inheriting `collections.abc.Mapping`. This patch implements the
__iter__() and __len__() methods so as to let `collections.abc.Mapping`
implement __contains__(), keys(), items(), values(), get(), __eq__(),
and __ne__(). Now you can do, for example:

    if 'next_prio' in event:
        print(event['next_prio'])

__iter__() keeps a set of yielded field names so as to avoid repeating
field names.

__len__() simply reuses __iter__().

`test_event.py` is updated to test the new features.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I2485e0df2373005fada0d0732c6269acbce5457c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3343
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>