Implement delay object and macro

[jeaye]

This will follow Clojure's delay (Delay.java). Community docs: https://clojuredocs.org/clojure.core/delay

We already have support for @ and deref, but force and delay don't exist yet.

1. Create a new object_type for delay (under reduced)
2. Add a case for delay under reduced in visit_object
3. Copy volatile.hpp into delay.hpp and implement deref
4. Add delay and force to clojure.core

[jianlingzhong]

will take this one.

[jianlingzhong]

https://github.com/jank-lang/jank/pull/123 should have completed this feature.

[jeaye]

Yep. In the PR, you can add "This closes #nnn" and Github will do it for us automatically.