Single point locations (equal boundaries) are troublesome, and we were
incorrectly ending the style in their case. Which results in an abort
in libtextstyle.
There is also a confusion between columns as displayed on the
screen (which take into account multibyte characters and tabulations),
and the number of bytes. Counting the screen-column
incrementally (character by character) is uneasy (because of multibyte
characters), and I don't want to maintain a buffer of the current line
when displaying the diagnostic. So I believe the simplest solution is
to track the byte number in addition to the screen column.
* src/location.h, src/location.c (boundary): Add the byte-column.
Adjust dependencies.
* src/getargs.c, src/scan-gram.l: Adjust.
* tests/diagnostics.at: Check zero-width locations.
Enable checking of styles even when libtextstyle is not installed.
* src/getargs.h, src/getargs.c (style_debug): New.
(getargs_colors): Set it when --style=debug.
* src/complain.c (begin_use_class, end_use_class): Use it.
* tests/diagnostics.at: New.
