Write a symbol macro let walkthrough

[gilch]

Related to #179. This would be another code-walking macro tutorial. Again, #173 would have to be done first. I think I've got that implemented and pushed up in a branch, but it's not tested. I probably need to try at least one of these to make sure they're working before I merge it in. I'm not sure I need both, but I'm also not sure which is easiest. I think this one might be easier, since I proposed to simplify the debugger to just stepping and inspecting locals, but I'd probably also have to demonstrate a usage for symbol macros. One that comes to mind would be lazy function parameters (like R's).