The extmap.ml file was taken (and extended) from ocaml 3.12 and has not
been updated since. Since then, ocaml map.ml has evolved and contains
some space optimizations.

The commit contains more changes than strictly needed. The objective is
to be as close as possible to map.ml from ocaml 4.04. After this patch,
the 'diff' wrt. map.ml  contains almost exclusively additions, and no
other changes.

Change-Id: I3e31f6068562e5e1c48f8426efc9ce4e2f5b6010