add foreign value wrapper

add foreign value wrapper

* libguile/foreign.h:
* libguile/foreign.c: New files, implementing simple wrappers around
  foreign values, such as those that one might link in dynamically from
  a library.

* libguile/tags.h (scm_tc7_foreign): Take a tc7 for foreign values.

* libguile.h:
* libguile/init.c: Add foreign.h to headers and init.

* libguile/print.c (iprin1): Add printer for foreign values.

* libguile/gc.c (scm_i_tag_name): Case for foreign values.
* libguile/goops.c (scm_class_of, create_standard_classes): Add a class
  for foreign values.

* libguile/evalext.c (scm_self_evaluating_p): Add case for foreign
  values.

* libguile/Makefile.am: Add foreign.[ch] to the build.