* tests/gpgscm/scheme-private.h (struct scheme): New fields for the
static integer cells.
* tests/gpgscm/scheme.c (_alloc_cellseg): New function.
(alloc_cellseg): Use the new function.
(MAX_SMALL_INTEGER): New macro.
(initialize_small_integers): New function.
(mk_small_integer): Likewise.
(mk_integer): Return a small integer if possible.
(_s_return): Do not free 'op' if it is a small integer.
(s_save): Use a small integer to box the opcode.
(scheme_init_custom_alloc): Initialize small integers.
(scheme_deinit): Free chunk of small integers.
* tests/gpgscm/scheme.h (USE_SMALL_INTEGERS): New macro.