Arrays: new and old keys

[David Bovill]

There were some unspecified limits on keys with the old array structures,
and even more with the GUI's that displayed them :) Array values could
always take binary values - so anything basically but the keys could not -
so for instance UTF8 unicode keys won't work. Does anyone know what exactly
is or isn't allowed for the old keys, and whether this still applies to the
new arrays?