Changed writer interface from a funtion to a [const] vtable struct.

lib/c/Makefile

lib/c/labcomm.c

lib/c/labcomm.h

lib/c/labcomm_dynamic_buffer_writer.c

lib/c/labcomm_dynamic_buffer_writer.h

lib/c/labcomm_fd_writer.c

lib/c/labcomm_fd_writer.h

lib/c/labcomm_private.h

lib/c/test/test_labcomm_basic_type_encoding.c

lib/c/test/test_labcomm_generated_encoding.c