Add debugging-metadata and tracing directly to OCaml traits-module

[ELLIOTTCABLE]

This PR implements an alternative to #117, making tracing-information non-optional (and thus requiring no build-system changes), instead requiring traits-implementations to be updated to accept the new tracing-information.

This is a breaking change for custom traits-implementations.

The built-in traits implementation use ocaml-trace, if present in the installed world (opam install trace), to generate tracing-spans for each SQL statement.

All of the additions are noops at runtime if the user does not have ocaml-trace installed, and doesn't use a custom traits implementation.

The diff in the generated output is as following:

@@ -7,6 +7,10 @@ module Make (T : Sqlgg_traits.M_io) = struct
   module IO = T.IO
 
   let get_something db ~user_id callback =
+    let __sqlgg_sql = ("SELECT DISTINCT(some_name)\n\
+FROM some_users\n\
+WHERE user_id = ?")
+    in
     let invoke_callback stmt =
       callback
         ~some_name:(T.get_column_Text stmt 0)
@@ -16,12 +20,14 @@ module Make (T : Sqlgg_traits.M_io) = struct
       T.set_param_Int p user_id;
       T.finish_params p
     in
-    T.select db ("SELECT DISTINCT(some_name)\n\
-FROM some_users\n\
-WHERE user_id = ?") set_params invoke_callback
+    T.select ~operation:"SELECT" ~span_name:"get_something" db __sqlgg_sql set_params invoke_callback
 
   module Fold = struct
     let get_something db ~user_id callback acc =

[ygrek]

• need to finish dune conditional

[ygrek]

lets put all meta information into one record { tables; span_name; operation }

[ygrek]

List.hd :sideeye: