Define basic IO abstractions, implement for FD

lib/include/io_abs.h

@@ -0,0 +1,17 @@
+#ifndef IO_ABS_H
+#define IO_ABS_H
+
+#include <stdlib.h>
+#include <stdint.h>
+
+struct io_reader {
+    void *ctx;
+    ssize_t (*read)(void *ctx, uint8_t *buf, size_t len);
+};
+
+struct io_writer {
+    void *ctx;
+    ssize_t (*write)(void *ctx, const uint8_t *buf, size_t len);
+};
+
+#endif