Add a chapter with a small introduction on debugging and profiling

[XVilka]

A lot of changes in debugging and profiling recently in OCaml world, many online manuals are obsolete. It would be highly beneficial for newcomers to know the basics of modern OCaml debugging and profiling, with an initial setup of the corresponding tools. My suggestion is to separate it from http://dev.realworldocaml.org/compiler-backend.html into distinct chapter, update the content - e.g. kill the gprof since it was removed in the current OCaml master. Also add the information about spacetime:

• https://caml.inria.fr/pub/docs/manual-ocaml/spacetime.html
• https://blog.janestreet.com/a-brief-trip-through-spacetime/
• https://discuss.ocaml.org/t/status-of-ocaml-spacetime/1455/5
• https://github.com/ocaml/ocaml/issues/7678

[XVilka]

A good reference article too https://github.com/ocaml-bench/notes/blob/master/profiling_notes.md, including the new statistical profiler.

There is also https://github.com/LexiFi/landmarks profiling library.