Changes

Add type Obj.raw_data and functions Obj.raw_field, Obj.set_raw_field

Some OCaml objects contain data that cannot be safely represented
as an OCaml value (type Obj.t).  For example, in no-naked-pointers mode,
this is the case for code pointers inside closures, and for the
"custom operations" pointers inside custom blocks.

This PR introduces a type Obj.raw_data (an alias for nativeint)
to encapsulate this data, and functions
Obj.raw_field / Obj.set_raw_field to read and write the "raw" contents
of fields of blocks.

Note: just like it is wrong to access code pointers and custom operations
using Obj.field / Obj.set_field, it is wrong to access regular fields
possibly containing pointers into the OCaml heap using
Obj.raw_field / Obj.set_raw_field.  The OCaml heap block can be
reclaimed or moved after its address was captured by Obj.raw_field.
Symmetrically, Obj.set_raw_field on a regular field bypasses the
write barrier of the GC.
 (commit: ec33006)

Use type Obj.raw_data to represent code pointers in the REPL trace facility

Using Obj.t is incorrect in no-naked-pointer mode, as it exposes
code pointers as OCaml values.
 (commit: c2db328)

Test intext.ml: do not use the caml_static_alloc primitive

This primitive (from runtime/obj.c) is being phased out because
it returns a naked pointer outside the OCaml heap.

Instead, for the test, use a statically-allocated buffer
that is never visible from OCaml.
 (commit: b0cd12d)

Remove the primitive functions caml_static_{alloc,free,resize}

These primitives are dangerous because they produce naked pointers
outside the OCaml heap, with a risk of "GC pointer confusion".
(After caml_free and a heap extension, the freed memory area can be
reallocated as part of the OCaml heap, causing the naked pointer to
become a bad heap pointer).

These primitives are not used anywhere in the core OCaml system
(in particular they are not exposed via the Obj module).
An OPAM-wide grep shows no uses there either.
 (commit: d6f9496)

Bootstrap after removal of caml_static_{alloc,free_resize} primitives

Follow-up to d6f949608
 (commit: 0a11f73)

Changes entry for #9655
 (commit: 0a2dacb)

Changes entry for #9675
 (commit: 2ad3a03)