cxxrtl: export wire attributes through the C API

[whitequark]

Among other things this allows retrieving source locations for wires in the simulation, e.g.:

void print_location_cb(void *, const char *name, struct cxxrtl_object *object, size_t parts) {
	printf("%s:\n", name);
	cxxrtl_attr_set attrs = object->attrs;
	if (cxxrtl_attr_type(attrs, "src") == CXXRTL_ATTR_STRING) {
		const char *src = cxxrtl_attr_get_string(attrs, "src");
		while (src) {
			char buf[256] = {0};
			const char *next = strchr(src, '|');
			if (!next) {
				printf("\t%s\n", src);
				src = NULL;
			} else {
				strncpy(buf, src, next - src);
				printf("\t%s\n", buf);
				src = next + 1;
			}
		}
	} else {
		printf("(missing!)\n");
	}
}

int main() {
	cxxrtl_handle top = cxxrtl_create_at(cxxrtl_design_create(), "toplevel");
	cxxrtl_reset(top);

	printf("Source location information for this simulation:\n");
	cxxrtl_enum(top, NULL, print_location_cb);

	return 0;
}