when OCAML_GC_STATS points to a filepath, write Gc stats into it

This interface is promoted by the operf-macro tool
  https://github.com/OCamlPro/operf-macro
which allows to run benchmarks of time and memory usage
of various OCaml programs.

Coq already has two ways to get Gc infos:
- the -m|--memory command-line flag prints the total heap words allocated
- the "Print Debug Gc" command prints much more information,
  but in a Coq-implementation-defined format that is not suitable
  for across-programs comparison
  (also an environment variable allows to profile Coq runs on any .v,
   in an non-intrusive way)

Note to the Github Robot:
This closes #75

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