Remove SMOB tutorial; update manual.

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* doc/ref/libguile-smobs.texi: Remove; this tutorial is superseded by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