term: lower maximum number of characters in a compose chain

Before the grapheme cluster segmentation work, we limited the number
of combining characters to base+5. I.e. 6 in total.

For a while now, we’ve had it bumped all the way up to 20. This was
the reason the unicode-random benchmark ran so much slower (i.e. cache
contention).

Looking at emoji’s, there are a couple that need 6 code points,
and *three* that needs 7.

Now, with the limit at 7 chars, and the new ‘width’ member, the
composed struct is 8 bytes larger than before.

terminal.h

@@ -85,7 +85,7 @@ struct damage {
 };
 
 struct composed {
-    wchar_t chars[20];
+    wchar_t chars[7];
     uint8_t count;
     int width;
 };