cenum->sexp stubs that don't throw
Sexp_ext module
Start populating Sexp_ext
add cstruct syntax decorator, `(sexp)`

now depends on https://github.com/mirage/ocaml-cstruct/pull/21
extend Monad_ext
rethink tracing -> implicit trace ids
hook up tracing
hardcore simplify
clean leftovers
first tracing
hook up tracing to lwt