Inline nodes in our layout tree have a position, so let's show it. By
centralizing the logic for this, block nodes now lose their redundant
'content-size' dump info which is already part of the box model dump.
The fix here was to stop using StringBuilder::append(char) when told to
append a code point, and switch to StringBuilder::append_code_point(u32)
There's probably a bunch more issues like this, and we should stop using
append(char) in general since it allows building of garbage strings.
