Added tests/eval.scm, `make-closure` and `secdctl` natives

1) added native function `make-closure`, which takes a list of commands
as a body of a function and makes a closure on the stack that may be
called.
2) added example of interpreting input in REPL loop.
3) fixed compilation of forms like `((if addition? + -) 3 2)` to make
forms like `((make-closure input))` possible.

4) added native function `secdctl`, which outputs information about SECD
machine state:
* `(secdctl 'free)` shows how many free cells are available;
* `(secdctl 'env)` prints the current environment.