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/oop/goops.scm

@@ -1,6 +1,6 @@
 ;;;; goops.scm -- The Guile Object-Oriented Programming System
 ;;;;
-;;;; Copyright (C) 1998-2003,2006,2009-2011,2013-2015,2017-2018,2021
+;;;; Copyright (C) 1998-2003,2006,2009-2011,2013-2015,2017-2018,2021,2025
 ;;;;   Free Software Foundation, Inc.
 ;;;; Copyright (C) 1993-1998 Erick Gallesio - I3S-CNRS/ESSI <eg@unice.fr>
 ;;;;
@@ -69,7 +69,7 @@
             <boolean> <char> <list> <pair> <null> <string> <symbol>
             <vector> <bytevector> <uvec> <foreign> <hashtable>
             <fluid> <dynamic-state> <frame> <vm> <vm-continuation>
-            <keyword> <syntax> <atomic-box>
+            <keyword> <syntax> <atomic-box> <thread>
 
             ;; Numbers.
             <number> <complex> <real> <integer> <fraction>
@@ -81,7 +81,7 @@
             ;; corresponding classes, which may be obtained via class-of,
             ;; once you have an instance.  Perhaps FIXME to provide a
             ;; smob-type-name->class procedure.
-            <promise> <thread> <mutex> <condition-variable>
+            <promise> <mutex> <condition-variable>
             <regexp> <hook> <bitvector> <random-state>
             <directory> <array> <character-set>
             <dynamic-object> <guardian> <macro>
@@ -1078,6 +1078,7 @@ slots as we go."
 (define-standard-class <uvec> (<bytevector>))
 (define-standard-class <array> (<top>))
 (define-standard-class <bitvector> (<top>))
+(define-standard-class <thread> (<top>))
 (define-standard-class <number> (<top>))
 (define-standard-class <complex> (<number>))
 (define-standard-class <real> (<complex>))
@@ -3525,7 +3526,6 @@ var{initargs}."
 ;;;
 
 (define <promise> (find-subclass <top> '<promise>))
-(define <thread> (find-subclass <top> '<thread>))
 (define <mutex> (find-subclass <top> '<mutex>))
 (define <condition-variable> (find-subclass <top> '<condition-variable>))
 (define <regexp> (find-subclass <top> '<regexp>))