sort: implement --debug

This adds a --debug flag, which, when activated, will draw lines below
the characters that are actually used for comparisons.

This is not a complete implementation of --debug. It should, quoting the man page
for GNU sort: "annotate the part of the line used to sort, and warn
about questionable usage to stderr". Warning about "questionable usage"
is not part of this patch.

This change required some adjustments to be able to get the range that
is actually used for comparisons. Most notably, general numeric comparisons
were rewritten, fixing some bugs along the lines.

Testing is mostly done by adding fixtures for the expected debug output of
existing tests.

Cargo.lock

@@ -2297,6 +2297,7 @@ dependencies = [
  "rayon",
  "semver",
  "smallvec 1.6.1",
+ "unicode-width",
  "uucore",
  "uucore_procs",
 ]