tests/gpgscm: Foreign objects support for TinySCHEME.
* tests/gpgscm/scheme-private.h (struct cell): Add 'foreign_object'.
(is_foreign_object): New prototype.
(get_foreign_object_{vtable,data}): Likewise.
* tests/gpgscm/scheme.c (enum scheme_types): New type.
(is_foreign_object): New function.
(get_foreign_object_{vtable,data}): Likewise.
(mk_foreign_object): Likewise.
(finalize_cell): Free foreign objects.
(atom2str): Pretty-print foreign objects.
(vtbl): Add new functions.
* tests/gpgscm/scheme.h (struct foreign_object_vtable): New type.
(mk_foreign_object): New prototype.
(struct scheme_interface): Add new functions.Patch from Thomas Munro,
https://sourceforge.net/p/tinyscheme/patches/13/
- Signed-off-by: Justus Winter <justus@g10code.com>