New foreign object facility, to replace SMOBs

* libguile/foreign-object.c:
* libguile/foreign-object.h:
* module/system/foreign-object.scm:
* test-suite/standalone/test-foreign-object-c.c:
* test-suite/standalone/test-foreign-object-scm: New files.

* test-suite/standalone/Makefile.am:
* module/Makefile.am:
* libguile/Makefile.am: Add new files.

* libguile.h: Add foreign-object.h.
* libguile/init.c (scm_i_init_guile): Call scm_register_foreign_object.
Add scm_make_foreign_object_0; optimize scm_make_foreign_object_n.

* libguile/foreign-object.c (scm_make_foreign_object_0): New function.
  (scm_make_foreign_object_n): Pre-fetch layout_chars.

* libguile/foreign-object.h: Add scm_make_foreign_object_0.
Avoid non-constant struct initializers

* libguile/foreign-object.c (scm_make_foreign_object_2):
  (scm_make_foreign_object_3): Avoid non-constant struct initializers.
Add more foreign object interfaces

* libguile/foreign-object.c:
* libguile/foreign-object.h (scm_make_foreign_object_1)
  (scm_make_foreign_object_2, scm_make_foreign_object_3)
  (scm_make_foreign_object_n): Change to take void * arguments, and to
  add a comment to the header indicating that these are convenience
  constructors.

* libguile/foreign-object.c:
* libguile/foreign-object.h (scm_foreign_object_unsigned_ref)
  (scm_foreign_object_unsigned_set_x): New functions, equivalent to the
  old scm_foreign_object_ref and scm_foreign_object_set_x.

* libguile/foreign-object.c:
* libguile/foreign-object.h (scm_foreign_object_signed_ref)
  (scm_foreign_object_signed_set_x): New functions taking
  scm_t_signed_bits.

* libguile/foreign-object.c:
* libguile/foreign-object.h (scm_foreign_object_ref)
  (scm_foreign_object_set_x): New functions that take void*.
Add foreign object documentation

* doc/ref/api-foreign-objects.texi:
* doc/ref/libguile-foreign-objects.texi: New files.

* doc/ref/guile.texi:
* doc/ref/Makefile.am: Link new files into docs.
Remove SMOB tutorial; update manual.

* doc/ref/libguile-smobs.texi: Remove; this tutorial is superseded by
  libguile-foreign-objects.

* doc/ref/libguile-foreign-objects.texi: Proofreading.

* doc/ref/libguile-program.texi: Update Dia examples to refer to foreign
  objects.

* doc/ref/libguile-concepts.texi (Garbage Collection): Update to
  accurately describe the BDW-GC, and to avoid reference to mark
  functions.

* doc/ref/guile.texi: Remove libguile-smobs, and reword API menu.

* doc/ref/api-utility.texi (Equality): Mention GOOPS instead of SMOBs.

* doc/ref/api-smobs.texi (Smobs): Describe as a legacy interface.
  Exhort readers against the writing of mark functions.

* doc/ref/api-foreign-objects.texi (Foreign Objects): Proofreading.

* doc/ref/api-control.texi (Catch): Fix ref to point to foreign
  objects.

* doc/ref/Makefile.am: Remove libguile-smobs.texi.
Remove doc/example-smob/

* doc/example-smob/: Remove example.  It's smaller than it used to be in
  Guile 1.8, and with foreign objects there's practically nothing
  interesting to test.

* doc/oldfmt.c: Remove this hoary thing.

* doc/README:
* doc/Makefile.am: Update.
Add new standalone tests to gitignore

* .gitignore: Add entries.
Prefer foreign objects over smobs in manual

* doc/ref/api-memory.texi (Memory Blocks): Recommend against
  scm_gc_free.  Refer to foreign objects instead of smobs.  Remove
  discussion of scm_must_malloc et al.

* doc/ref/guile.texi (API Reference): Rename SMOB menu item.

* doc/ref/libguile-snarf.texi (Function Snarfing): Update example to not
  refer to smobs.
* doc/ref/tools.texi (How guile-snarf works): Likewise.