gpgscm: Remove framework for immediate values.

* tests/gpgscm/scheme.c (IMMEDIATE_TAG): Remove macro.
(is_immediate): Likewise.
(set_immediate): Likewise.
(clr_immediate): Likewise.
(enum scheme_types): Set the LSB in every value.
(fill_vector): Adapt.
(vector_elem): Likewise.
(set_vector_elem): Likewise.
(mark): Likewise.
(gc): Test for the LSB to tell typeflags apart from pointers stored in
the same memory location.

Supporting immediate values would require invasive changes to the
interpreter and is likely not worth the trouble. On the other hand,
tagging pointers in vectors complicated the hash table implementation
needlessly. Therefore, I remove this again.

This fixes a crash on big endian architectures.

• GnuPG-bug-id: T2996
• Signed-off-by: Justus Winter <justus@g10code.com>