Propagation compiler passes using meander

[bsless]

Following up on #212, the next step is copy and constant propagation, which stumped me

Generally, I would want to rewrite something like:

(let [x 1 y 2] (+ x y)
(+ 1 2)

In a generic manner with a top down strategy.

Writing a pattern which matches the biding was not difficult, and I managed to capture the bound name and the initial value

The problem is writing something which would generically walk over the body, then substitute AST nodes which reference a scoped variable with the constant nodes. I'm guessing I need something like cata, but even then, I'd have to specify all possible "shapes" the data could take in order to walk it. While the AST spec is rather small, writing out all the possible productions is verbose and slightly tedious.

Any ideas how this could be simplified?