Allow custom folding per form

[cursive-ghost]

It would be useful to be able to specify for a particular form how it should be folded. For example, a let could be folded to just show the variables it defines, defn could show the available arities and parameters and so on.

It would also be desirable for forms to be able to identify their body forms, so that these could be folded if they were data structures, e.g. a function which returns a large hiccup structure.