Change to be less sloppy as regards functions without prototypes

* libguile/gsubr.h (scm_t_subr_0, scm_t_subr_1, etc): New precise
typedefs.
(SCM_AS_SUBR): Use C11's _Generic to cast subrs to the generic subr
type, while also producing a warning/error if the function isn't
compatible.
(SCM_DEFINE_GSUBR, SCM_PRIMITIVE_GENERIC, SCM_DEFINE_PUBLIC)
(SCM_DEFINE_STATIC, SCM_PROC, SCM_REGISTER_PROC, SCM_GPROC): Use
SCM_AS_SUBR.
* libguile/gsubr-internal.h (scm_t_subr_with_thread_0)
(scm_t_subr_with_thread_1, etc): New precise typedefs.
(SCM_AS_SUBR_WITH_THREAD): Like SCM_AS_SUBR.
* libguile/gsubr.c (scm_apply_subr): Cast callee to the right type
before calling.
* libguile/hash.c (floor): Remove weird unused declaration.
* libguile/init.c (scm_boot_guile): Fix type of main_func in definition.
* libguile/jit.c: Fix type of enter_mcode.
* libguile/smob.c (apply_0, apply_1, apply_2, apply_3): Cast callee to
right type.
(scm_smob_trampoline): Use SCM_AS_SUBR.
* libguile/smob.h (SCM_SMOB_APPLY): Use SCM_AS_SUBR.
* libguile/backtrace.c:
* libguile/control.c:
* libguile/dynl.c:
* libguile/eval.c:
* libguile/exceptions.c:
* libguile/expand.c:
* libguile/finalizers.c:
* libguile/fluids.c:
* libguile/fports.c:
* libguile/frames.c:
* libguile/gc.c:
* libguile/load.c:
* libguile/loader.c:
* libguile/macros.c:
* libguile/memoize.c:
* libguile/pairs.c:
* libguile/poll.c:
* libguile/ports.c:
* libguile/posix.c:
* libguile/rdelim.c:
* libguile/rw.c:
* libguile/vm.c: Adapt scm_c_make_gsubr / scm_c_define_gsubr callers to
use SCM_AS_SUBR.

libguile/macros.c

@@ -252,5 +252,6 @@ scm_init_macros ()
 #include "macros.x"
 
   syntax_session_id = fresh_syntax_session_id();
-  scm_c_define_gsubr ("syntax-session-id", 0, 0, 0, scm_syntax_session_id);
+  scm_c_define_gsubr ("syntax-session-id", 0, 0, 0,
+                      SCM_AS_SUBR (scm_syntax_session_id));
 }