Memory leaks in Rust implementation #709

Circular references using Rc in Rust causes memory leaks! An environment can reference a function and a function can reference an environment, creating a circular reference. Possible fixes for the issue could be:

• Cloning the outer environment when a function is created instead of keeping a reference to it, this could causes issues with referencing variables before assignment e.g if the environment gets mutated the functions own copy of the environment won't be changed.
• Using a Weak reference instead of Rc, this could cause complications with function currying and referencing environments that have been dropped.
• Implementing a garbage collector. The dumpster crate looks promising as a drop-in replacement for Rc.

I have already added the cloning outer reference method into my own mal rust implementation.

Or since the impls are mainly there for demonstration purposes it might not be worth the added complexity of fixing this issue.