Changes

configure: add a NAKED_POINTERS parameter to Makefile.config

We have NAKED_POINTERS=false if configured with --disable-naked-pointers,
NAKED_POINTERS=true otherwise.
 (commit: 41227a1)

ocamltest: add the "naked_pointers" action

It succeeds if the runtime system supports naked pointers,
and fails otherwise (e.g. if configured with --disable-naked-pointers).
 (commit: 133102c)

Revise testing of Obj.reachable_words

Part of the test makes sense only if the runtime system supports
naked pointers and has a page table to distinguish major heap pointers
from out-of-heap pointers.

This part is split off in a new test, lib-obj/reachable_words_np.ml,
conditionalized on "naked_pointers".
 (commit: ccb7829)

Reimplement Obj.reachable_word using a hash table to detect sharing

The previous implementation (caml_obj_reachable_words in runtime/obj.c)
was using the mark bits of block headers to detect sharing.

This is not compatible with Multicore OCaml.

This commit reimplements caml_obj_reachable_words to detect sharing
with a hash table of addresses already seen.

The implementation reuses the hash table used to detect sharing
during marshaling (commit 67ada54ce), and other bits of the marshaler,
hence the function caml_obj_reachable_words was moved to runtime/extern.c.

In no-naked-pointers mode, to anticipate the disappearance of the page
table, statically-allocated blocks cannot be treated specially and will
be counted towards the size.  This change of semantics is mentioned
in the documentation for Obj.reachable_words.
 (commit: e52bf32)

Changes entry for #9678
 (commit: 7e4ef62)