Write a toAST semantic action

[nfischer]

So far, I've been transpiling straight from a CST. This has some nice features, since I can sometimes preserve whitespace and comments, which I think are still significant parts of the code.

But I'd like to eventually publish the grammar in its own module, once it's sufficiently general and stable. When I do this, I'd like to have a toAST operation in the package, that's somewhat similar to https://github.com/vorpaljs/bash-parser.