bt2: field.py: refactor field comparison

bt2: field.py: refactor field comparison

Changed:

* In _NumericField._extract_value(), do not check the `False`, and
  `True` special cases: check that the parameter is an instance of the
  `bool` type to return a boolean object.

* In _ArrayField.__eq__(), be more strict: expect that the parameter is
  a sequence object, not just an iterable object. Before this, it was
  possible to compare an array field to an ordered dict with keys equal
  to the array field content, and this seems wrong as:

      collections.OrderedDict((('A', 23), ('B', 42))) == ['A', 'B']

  is false. An ordered dict is not a sequence.

* In _StructureField.__eq__(), be more strict: expect that the parameter
  is a mapping object, not just an iterable and indexable object. The
  reason is similar to the _ArrayField.__eq__() case above. This should
  be enough to compare to another structure field or to a dict (or
  ordered dict).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I17f33c24e9dea526e59b5058235d57facb51cfbf
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1564
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>