fix #K4019 - "can not"

Report suggested that "can not" should be "cannot".  Both forms are
acceptable.  This switches them to use contractions for various "You
<verb> not subject" phrases:  "You can't subject", "hadn't", and so
forth.  Not exhaustively tested; there may be some sentences where the
informal contraction makes things worse rather than better.

The goal here was compactness rather than efficiency since the code
involved doesn't execute very often.

src/insight.c

@@ -127,12 +127,34 @@ enlght_out(const char *buf)
 }
 
 static void
-enlght_line(const char *start, const char *middle, const char *end,
-            const char *ps)
+enlght_line(
+    const char *start,
+    const char *middle,
+    const char *end,
+    const char *ps)
 {
+#ifndef NO_ENLGHT_CONTRACTIONS
+    static const struct contrctn {
+        const char *twowords, *contrctn;
+    } contra[] = {
+        { " are not ", " aren't " },
+        { " were not ", " weren't " },
+        { " have not ", " haven't " },
+        { " had not ", " hadn't " },
+        { " can not ", " can't " },
+        { " could not ", " couldn't " },
+    };
+    int i;
+#endif
     char buf[BUFSZ];
 
     Sprintf(buf, " %s%s%s%s.", start, middle, end, ps);
+#ifndef NO_ENLGHT_CONTRACTIONS
+    if (strstri(buf, " not ")) { /* TODO: switch to libc strstr() */
+        for (i = 0; i < SIZE(contra); ++i)
+            (void) strsubst(buf, contra[i].twowords, contra[i].contrctn);
+    }
+#endif
     enlght_out(buf);
 }