Unify AST with Smoosh

Right now ast.ml and shim.ml in smoosh are out of sync. It'd be great if there were only one copy of the code that converts dash's data structures to OCaml trees.

Some trickiness: it'd be ideal to not have to port over all of the smoosh runtime terms, just the actual parseable ones. Doing so would add an extra layer of conversion, though, which is probably pretty costly. It should be possible to functorize the dash conversion: given some functions that generate appropriate values of some abstract type, we'll call them in the right order.