basic tracing module
add sexplib
sexp core.ml
cert sexps
config sexps
Cstruct_with_state -> Cstruct_s

Soo much shorter..
sexp state
tracing goes sexp
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
fixup build files