Changes

callback.c: register the bytecode fragment used for callbacks

So that the return address pushed on the bytecode interpreter stack
is correctly recognized as a code pointer.
 (commit: f8970c4)

Revised linking of trap frames in bytecode interpreter stack

Rather than storing a pointer to the previous frame in the Trap_link
field of the current frame, store the distance (pointer difference)
between the current frame and the previous frame, tagged as an OCaml
integer.

Using a tagged integer instead of a raw pointer means fever problems
later with strict no-naked-pointer support.

Using a distance rather than an absolute address simplifies
the code that resizes the stack.
 (commit: 53d55f2)

Revised scanning of bytecode interpreter stack

In no-naked-pointers mode, recognize and skip code pointers present in
the stack of the bytecode interpreter.  This is needed only for
the scan at beginning of a major GC cycle, not for the scan done
at every minor GC.
 (commit: 9476d5d)

major_gc.c: use no-naked-pointers mode even in bytecode

Earlier, no-naked-pointers mode was effective only in native code.
 (commit: 4a953a2)

interp.c: remove two assertions that cannot be checked without the page table

`!Is_in_value_area(pc)` is always false if we turn the page table off.
A better check would be `caml_find_code_fragment_by_pc(pc) != NULL`,
but I feel this is too costly even for the debug mode of the interpreter.
 (commit: 552bc3e)