Review and fix memory model

[dibyendumajumdar]

Goptical's memory model is complex and hard to debug. Various objects point to each other and it is impossible to be sure that something isn't broken.

We should probably implement an indirect object pointer approach where all object references go via a lookup - this can then ensure that if an object is no longer present all users will get an error next time they try to de-reference.

Alternate simpler model is to just use an arena allocator and not release individual objects at all, but only the whole system at a go.