Add tutorial for macros

[gilch]

I could certainly walk through re-implementations of some of the basic macros. Should probably include nested templates, like defmacro/g/defmacro!. See if we can make the Lisp-2 macro work.

[gilch]

The first lesson is up #78.

[gilch]

There are still a number of TODOs in the macro tutorial source. Some of them seem outdated. The first two already cover a lot of ground, so additional lessons would have diminishing returns, but there are a few important techniques that still deserve mention. Some advanced macro techniques also applicable to Clojure or Common Lisp are perhaps already better covered elsewhere. Perhaps at most they deserve a link. However, some techniques are unique enough to Hissp (or Lissp) that they merit further tutorials.

In particular:

• The use of Extras. Maybe not stable enough, and already covered in the quickstart. Unique to Lissp.
• Metaprogramming the _macro_ object by implementing its __getattr__ (e.g. the Lisp-2 technique). Unique to Hissp.
• Pre-expansion and code walking. The original plan was for a code-walking yield macro. I'm no longer certain if that is feasible, but we might be able to do better than Ensue. I limited Hissp to two special forms in part to make this kind of thing easier.
• Destructuring fn. I put a lot of thought into lambda's parameters tuple to make this work better. This is pretty much unique to Hissp.

And easier lessons may be required to lead up to these.