Closures (and garbage collection for environments, which is required by closures...

* Add proper reference counting to environments

* It has become clear we need a garbage collector for environments before we can implement closures

* Allow line comments starting with #

* Allocate Environments from a garbage-collected EnvironmentPool

* Wrap closure functions in a struct in preparation for storing the defining environment with them

* Move the instantiation of closures into where they are created

* Fixed a bug in the mark/sweep algorithm:

* It would infinitely recurse in the case of cycles, because nothing was
  checking that Environments had already been marked.
* Simply exiting on self->mark == mark would fix this, but introduce a
  separate, worse bug where recursion wouldn't reach some live objects,
  since some objects might already be marked with the current mark due
  to previous cycles.
* To fix, I introduced a pass over the pool that marks everything false
  so we can reliably assume that self->mark == true means that the
  environment has been marked in the current GC round. It's slower than
  I wanted, but it's better to do the correct thing slowly than the
  wrong thing quickly.

* Store the defining environment on closures and GC it appropriately

* Test a simple case of closures and fix the discovered errors

* Add a test that explicitly demonstrates freeing a cycle

* Allow user-defined functions to take arguments

* Another closure example