make aeson instances agree with themselves

This cleans up all the old rubble that came from the Tiled package I
originally took from hackage. It now uses generics instead of
implementing all the ToJSON and FromJSON instances by hand, and
(deserialize . serialise) will now actually return a (semantically)
equivalent json.

It'll now also reject keys that it doesn't know, which required adding
some in several places which the tiled package didn't know about (or
which were introduced after it was originally written, dunno).

Several more Maybes are required now, to represent the difference
between e.g. empty lists and on set value, which does make the code
slightly weirder in other places …