Rename scm_i_thread to scm_thread

* libguile/scm.h (struct scm_thread, scm_thread): Rename from
  scm_i_thread.
* libguile/deprecated.h (scm_i_thread): Add deprecated typedef.
* libguile/threads.h: Adapt to renaming.
* libguile/intrinsics.h:
* libguile/scmsigs.h:
* libguile/cache-internal.h: Remove threads.h includes; unnecessary with
  the forward decl.
* libguile/continuations.h:
* libguile/gc-inline.h:
* libguile/async.h: Adapt scm_thread type name change.
* libguile/async.c:
* libguile/continuations.c:
* libguile/control.c:
* libguile/dynstack.c:
* libguile/dynwind.c:
* libguile/eval.c:
* libguile/finalizers.c:
* libguile/fluids.c:
* libguile/gc.c:
* libguile/intrinsics.c:
* libguile/load.c:
* libguile/memoize.c:
* libguile/print.c:
* libguile/read.c:
* libguile/scmsigs.c:
* libguile/script.c:
* libguile/stackchk.c:
* libguile/stacks.c:
* libguile/symbols.c:
* libguile/threads.c:
* libguile/throw.c:
* libguile/vm-engine.c:
* libguile/vm.c: Adapt to type name change, and add additional includes
  as needed.

libguile/cache-internal.h

@@ -27,7 +27,6 @@
 
 #include "libguile/gc.h"
 #include "libguile/hash.h"
-#include "libguile/threads.h"
 
 
 /* A simple cache with 8 entries.  The cache entries are stored in a