Format Locations according to GNU conventions

There's hardly a reason to invent another format for error locations.
Using the GNU format has the advantage that Emacs and other tools
support them out of the box.

* basics/_location.sml (to_string, extract_components): Use
FILE:LINE1.COLUMN1-LINE2.COLUMN2 instead of
FILE:LINE1,COLUMN1 to FILE:LINE2,COLUMN2 to comply with GNU standards.