Factor out a portable Scheme layer which could target different implementations

While Chez gives us a good overall target for Clojure, it makes a few architectural assumptions, like being able to write instructions to a section of memory and then execute that code, that do not hold for targets like WASM which considers such behavior a security concern. It would be useful to decouple the host layer from Chez and make it easy to port to different Scheme runtimes which would give us access to things like interactive development for WASM via Guile. Since host code is already separate from the core language implementation, there's some more refactoring that would be needed, but it wouldn't be difficult to isolate Chez specific bits, and build a general Scheme layer on top.