* tests/gpgscm/scheme-private.h (struct scheme): Remove fixed-size
arrays for cell segments, replace them with a pointer to the new
'struct cell_segment' instead.
* tests/gpgscm/scheme.c (struct cell_segment): New definition.
(_alloc_cellseg): Allocate the header within the segment, return a
pointer to the header.
(_dealloc_cellseg): New function.
(alloc_cellseg): Insert the segments into a list.
(_get_cell): Allocate a new segment if less than a quarter of
CELL_SIGSIZE is recovered during garbage collection.
(initialize_small_integers): Adapt callsite.
(gc): Walk the list of segments.
(scheme_init_custom_alloc): Remove initialization of removed field.
(scheme_deinit): Adapt deallocation.