Move thread bindings to (ice-9 threads)

* libguile/init.c (scm_i_init_guile): Don't call scm_init_thread_procs.
* libguile/threads.c (scm_init_ice_9_threads): Rename from
  scm_init_thread_procs, make static.
  (scm_init_threads): Register scm_init_thread_procs extension.
* libguile/threads.h (scm_init_thread_procs): Remove decl.
* module/ice-9/boot-9.scm: Load (ice-9 threads), so that related side
  effects occur early.
* module/ice-9/deprecated.scm (define-deprecated): Fix to allow
  deprecated bindings to appear in operator position.  Export deprecated
  bindings.
  (define-deprecated/threads, define-deprecated/threads*): Trampoline
  thread bindings to (ice-9 threads).
* module/ice-9/futures.scm: Use ice-9 threads.
* module/ice-9/threads.scm: Load scm_init_ice_9_threads extension.
  Reorder definitions and imports so that the module circularity
  with (ice-9 futures) continues to work.
* module/language/cps/intmap.scm:
* module/language/cps/intset.scm:
* module/language/tree-il/primitives.scm: Use (ice-9 threads).
* module/language/cps/reify-primitives.scm: Reify current-thread
  in (ice-9 threads) module.
* module/srfi/srfi-18.scm: Use ice-9 threads with a module prefix, and
  adapt all users.  Use proper keywords in module definition form.
* test-suite/tests/filesys.test (test-suite):
* test-suite/tests/fluids.test (test-suite):
* test-suite/tests/srfi-18.test: Use ice-9 threads.
* NEWS: Add entry.
* doc/ref/api-scheduling.texi (Threads): Update.
* doc/ref/posix.texi (Processes): Move current-processor-count and
  total-processor-count docs to Threads.

module/language/cps/intmap.scm

@@ -34,6 +34,7 @@
   #:use-module (srfi srfi-9)
   #:use-module (srfi srfi-9 gnu)
   #:use-module (ice-9 match)
+  #:use-module ((ice-9 threads) #:select (current-thread))
   #:export (empty-intmap
             intmap?
             transient-intmap?

module/language/cps/intset.scm

@@ -29,6 +29,7 @@
   #:use-module (srfi srfi-9)
   #:use-module (srfi srfi-9 gnu)
   #:use-module (ice-9 match)
+  #:use-module ((ice-9 threads) #:select (current-thread))
   #:export (empty-intset
             intset?
             transient-intset?

module/language/cps/reify-primitives.scm

@@ -79,6 +79,7 @@
       make-atomic-box atomic-box-ref atomic-box-set!
       atomic-box-swap! atomic-box-compare-and-swap!)
      '(ice-9 atomic))
+    ((current-thread) '(ice-9 threads))
     ((class-of) '(oop goops))
     ((u8vector-ref
       u8vector-set! s8vector-ref s8vector-set!

module/language/tree-il/primitives.scm

@@ -21,6 +21,7 @@
 (define-module (language tree-il primitives)
   #:use-module (system base pmatch)
   #:use-module (ice-9 match)
+  #:use-module (ice-9 threads)
   #:use-module (rnrs bytevectors)
   #:use-module (system base syntax)
   #:use-module (language tree-il)