Similarly to %-formatting, let's add support for .format(). This is
easier to do because we can leverage string.Formatting and hook into the
process by proper subclassing. We do not need to implement parsing and
need to only customize handling of 's' and 'r' specifiers.

For testing we mostly reuse existing tests for %-formatting by amending
them a bit to exercise both %-formatting and format-formatting at the
same time: by converting %-format specification into corresponding
{}-format specification and verifying formatting result for that to be
as expected.

Some explicit tests for {}-style .format() are also added.