Add lenses

- this adds the lens framework, although there are currently no interesting
  lenses.  display_args.c was mostly moved to lens_default.c
diff --git a/printf.c b/printf.c
index a2b058b..34d2813 100644
--- a/printf.c
+++ b/printf.c
@@ -186,6 +186,7 @@
 	enum arg_type elt_type = ARGTYPE_UNKNOWN;
 	char len_buf[25] = {};
 	size_t len_buf_len = 0;
+	struct lens *lens = NULL;
 
 	for (; self->ptr < self->end; ++self->ptr) {
 		if (!self->percent) {
@@ -321,6 +322,9 @@
 				    len_buf, len_buf_len, infop) < 0)
 			return -1;
 
+		infop->lens = lens;
+		infop->own_lens = 0;
+
 		return 0;
 	}