grow_realloc.
(nettle_buffer_clear): Rewritten to use buffer->realloc.

* buffer.h (struct nettle_buffer): REplaced the GROW function
pointer with a nettle_realloc_func pointer and a void
(NETTLE_BUFFER_GROW): Deleted macro, use function instead.

Rev: src/nettle/buffer.c:1.2
Rev: src/nettle/buffer.h:1.3