cpp-common/bt2c: make CStringView convertible from std::string

cpp-common/bt2c: make CStringView convertible from std::string

This will make it possible to have functions that accept CStringViews:

    void foo(CStringView);

and pass in std::strings seamlessly:

    foo(myStdStr);

Change-Id: I76dc55438c2cd87f77ddec421dff7f119e53eff5
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11688
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>