It is possible for that variable to not be set if `awful` was
never loaded.

There was no "real" unsolvable GC issues, but at least for Lua
5.2 and 5.3, it wasn't possible to GC a wibox anymore. This
commit unwind the circular references with 3 new weak refs. This
is enough to get a specific test to pass on Lua 5.3. It's voodoo,
but this was actually a pretty bad leak. There's other according
to some test I wrote, but that's one.

As for to_string, it appears to be accidental refactoring oversight.

When a drawable has a pending redraw in a delayed call and the
drawin is GCed (which was impossible because it was broken
until the last commit), it crashes.

When you run this outside of the test runner, it works, but not
with it. Anyway, it adds the regression tests for the previous
crash, so it's worth having.