1.0.8.42: Fix some source-location bugs, alternate sb-cover annotation mode

1.0.8.42: Fix some source-location bugs, alternate sb-cover annotation mode

         * Bug introduced in the code coverage commit caused sldb-show-source
           to highlight the wrong forms in many cases.
         * Changes in 1.0.6.23 caused branch forms to be not annotated as
           such in sb-cover.
         * Add a new annotation mode to sb-cover, which basically uses
           the source location information of the car of the form instead
           of using the information for the form itself. This mode shows
           explicitly which forms have been instrumented. According to
           a #lisp poll, the old version looks nicer, so it's been retained
           as the default.