Opening this as a followup to #304 and #310 which added basic OCaml 4.08 compat, but didn't support the new features.

From @trefis, Tsig_typesubst and Tsig_modsubst occurrences will result in unresolved references, so we need testcases that exercise the new features and support for resolving them.

(Note that as of #304, OCaml 4.08 can be used with odoc for resolving existing code that doesn't use the new 4.08 features).