gpgscm: Implement tags.

* tests/gpgscm/opdefines.h: Add opcodes to create and retrieve tags.
* tests/gpgscm/scheme.c (T_TAGGED): New macro.
(mk_tagged_value): New function.
(has_tag): Likewise.
(get_tag): Likewise.
(mark): Mark tag.
(opexe_4): Implement new opcodes.
* tests/gpgscm/scheme.h (USE_TAGS): New macro.

Tags are similar to property lists, but property lists can only be
attached to symbols. Tags can not be attached to an existing object,
but a tagged copy can be created. Once done, the tag can be
manipulated in constant time.

Using this during parsing will enable us to produce meaningful error
messages.

• Signed-off-by: Justus Winter <justus@g10code.com>