bt2: add `IntegerRangeSet` const classes

Outside of simple rename and splitting, this commit changes the `__eq__`
methods so that it's possible to compare const and non-const objects
together.

I also renamed the `_range_type` to `_range_pycls` to follow what is
done in other files and `_borrow_range_by_index_ptr` to
`_borrow_range_ptr_by_index` to increase readability.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I04b4027d297d088c6531be04681598653601579d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2118
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>