Define recursive Heapster shapes from recursive Rust types

[eddywestbrook]

This updates the heapster_define_rust_type command, that defines Heapster shapes from Rust types, so that it can handle recursive Rust types, such as the List type

pub enum List<X> { Nil, Cons (X,Box<List<X>>) }

Note: this code represents a branch of both saw-script and heapster-saw that was not merged before Heapster was merged into saw-script master.

[ChrisEPhifer]

Everything looks good to me! Assuming this all builds / runs successfully on our rust_data example(s), this should be good to merge in :)