traces: use colors for the semantic values

This makes reading the trace slightly easier.  It would be very nice
to highlight the "big steps", especially reductions.  But this is a
private experiment: do not use it.

* data/diagnostics.css (value): New.
* src/parse-gram.y: Use no delimiters and no c quotation for strings
to facilitate debugging.
(tron, troff, TRACE): New.
Not very elegant, but until there is support for printf-formats in
libtextstyle, it shall be enough.

data/diagnostics.css

@@ -6,3 +6,5 @@
 .warning   { color: purple; }
 .error     { color: red; }
 .note      { color: cyan; }
+/* Semantic values in Bison's parser traces.  */
+.value     { color: green; }