lui.el: Fix some compile errors.

The call to `help-xref-interned` was missing the required library,
and `redraw-modeline` is an obsolete alias for `force-mode-line-update`,
which already exists in 23.4, so can be used safely without
backward compatibility issues.

Fixes #45.
circe-duration-string: Rewrite.

The old code was pretty buggy and could return the empty string in
a few corner cases. The new one should be correct.

Added some tests to make sure it does.