add buffer_fromarray, buffer_frombuf

buffer/buffer_fromarray.c

@@ -0,0 +1,9 @@
+#include "array.h"
+#include "buffer.h"
+
+void buffer_fromarray(buffer* b,array* a) {
+  if (array_failed(a))
+    buffer_frombuf(b,NULL,0);
+  else
+    buffer_frombuf(b,array_start(a),a->initialized);
+}