Summary:
Add support to ppx_trace to rewrite `[%trace]` into a call to the
corresponding `Trace` function:
```lang=ocaml
val trace :
?call:(pf -> unit)
-> ?retn:(pf -> 'a -> unit)
-> ?rais:(pf -> exn -> Printexc.raw_backtrace -> unit)
-> string
-> string
-> (unit -> 'a)
-> 'a
(** [trace ~call ~retn ~rais mod_name fun_name k] either simply invokes
[k ()], when not enabled, or else increases the indentation level and
emits the [call] message, then invokes [k ()], then decreases the
indentation level and either emits the [retn] or [rais] message,
depending on whether [k ()] returned normally or exceptionally. *)
```
The main motivation over the existing `Trace.call` and `Trace.retn` is
that by packaging them together, unhandled exceptions can be treated
better.
Reviewed By: jvillard
Differential Revision: D23636200
fbshipit-source-id: f61c267fd
Josh Berdine
7712de24e5