Use Whippet API to boot threads

* libguile/scm.h (scm_tc7_thread): Give threads their own tc7.
* libguile/threads.h (struct scm_thread): Add a tag, so that struct
thread can be a SCM directly.  Add a struct gc_mutator* member.
(scm_thread_handle): New inline function.
(SCM_I_IS_THREAD, SCM_I_THREAD_DATA, SCM_VALIDATE_THREAD): Update to use
tc7 instead of SMOB tags.

* libguile/continuations.c (scm_i_with_continuation_barrier)
* libguile/finalizers.c (queue_finalizer_async)
* libguile/jit.c (compile_current_thread)
* libguile/threads.c (block_self, guilify_self_2)
(lock_mutex, unlock_mutex, timed_wait scm_current_thread)
(scm_all_threads)
* libguile/vm-engine.c (current-thread): Use scm_thread_handle instead
of thread->handle.

* libguile/evalext.c (scm_self_evaluating_p):
* libguile/goops.c (class_thread, scm_class_of, scm_sys_goops_early_init)
* libguile/print.c (iprin1)
* module/language/cps/compile-bytecode.scm (compile-function)
* module/oop/goops.scm (<thread>)
* module/system/base/types.scm (cell->object)
* module/system/base/types/internal.scm (heap-tags)
* module/system/vm/assembler.scm: (emit-thread?): Adapt to
scm_tc7_thread.

* libguile/gc-internal.h: Move init functions that take "struct
gc_stack_addr" here, so that internal Whippet uses don't cause Whippet
to be added to public headers.
* libguile/gc.c (scm_storage_prehistory): Take struct gc_stack_addr as
arg, and pass to gc_init.  Return a mutator pointer.
* libguile/init.c (scm_i_init_guile): Pass mutator and stack base to GC
and thread init routines.
* libguile/threads.c (scm_trace_dynstack, scm_trace_thread)
(scm_trace_thread_mutator_roots): New infra for marking threads in terms
of Whippet API.
* libguile/threads.c (guilify_self_1): Since we don't use a separate GC
kind for threads any more, and thread marking is keyed off
gc_mutator_set_roots, we can avoid some of the gnarly synchronization.
(on_thread_exit): Arrange to gc_finish_for_thread.
(scm_i_init_thread_for_guile): Use gc_init_for_thread.
(init_main_thread, with_guile, scm_i_with_guile): Use Whippet API.
(scm_threads_prehistory): Take main-thread mutator and the stack base as
arguments.
* libguile/vm.c (scm_trace_vm): Rework in terms of Whippet API.
* libguile/whippet-embedder.h (gc_trace_mutator_roots): Arrange to trace
the current mutator's SCM thread object.
* libguile/trace.h: New file, to declare implementations of trace
routines.
* libguile/Makefile.am (noinst_HEADERS): Add trace.h.

module/system/base/types.scm

@@ -1,5 +1,5 @@
 ;;; 'SCM' type tag decoding.
-;;; Copyright (C) 2014, 2015, 2017, 2018, 2022 Free Software Foundation, Inc.
+;;; Copyright (C) 2014, 2015, 2017, 2018, 2022, 2025 Free Software Foundation, Inc.
 ;;;
 ;;; This library is free software; you can redistribute it and/or modify it
 ;;; under the terms of the GNU Lesser General Public License as published by
@@ -417,6 +417,8 @@ using BACKEND."
           (((_ & #x7f = %tc7-bytevector) len address)
            (let ((bv-port (memory-port backend address len)))
              (get-bytevector-n bv-port len)))
+          (((_ & #x7f = %tc7-thread))
+           (inferior-object 'thread address))
           ((((len << 8) || %tc7-vector))
            (let ((words  (get-bytevector-n port (* len %word-size)))
                  (vector (make-vector len)))

module/system/base/types/internal.scm

@@ -1,5 +1,5 @@
 ;;; Details on internal value representation.
-;;; Copyright (C) 2014, 2015, 2017, 2018, 2020, 2021 Free Software Foundation, Inc.
+;;; Copyright (C) 2014, 2015, 2017, 2018, 2020, 2021, 2025 Free Software Foundation, Inc.
 ;;;
 ;;; This library is free software; you can redistribute it and/or modify it
 ;;; under the terms of the GNU Lesser General Public License as published by
@@ -51,6 +51,7 @@
             %tc7-program
             %tc7-vm-continuation
             %tc7-bytevector
+            %tc7-thread
             %tc7-weak-set
             %tc7-weak-table
             %tc7-array
@@ -147,7 +148,7 @@
   (program          program?               #b1111111       #b1000101)
   (vm-continuation  vm-continuation?       #b1111111       #b1000111)
   (bytevector       bytevector?            #b1111111       #b1001101)
-  ;;(unused         unused                 #b1111111       #b1001111)
+  (thread           thread?                #b1111111       #b1001111)
   (weak-set         weak-set?              #b1111111       #b1010101)
   (weak-table       weak-table?            #b1111111       #b1010111)
   (array            array?                 #b1111111       #b1011101)

module/system/vm/assembler.scm

@@ -1,6 +1,6 @@
 ;;; Guile bytecode assembler
 
-;;; Copyright (C) 2001, 2009-2023 Free Software Foundation, Inc.
+;;; Copyright (C) 2001, 2009-2023, 2025, 2025 Free Software Foundation, Inc.
 ;;;
 ;;; This library is free software; you can redistribute it and/or
 ;;; modify it under the terms of the GNU Lesser General Public
@@ -132,6 +132,7 @@
             emit-program?
             emit-vm-continuation?
             emit-bytevector?
+            emit-thread?
             emit-weak-set?
             emit-weak-table?
             emit-array?