os: add pub fn read_bytes and pub fn read_bytes_at

vlib/os/os_test.v

@@ -33,6 +33,42 @@ fn test_write_and_read_string_to_file() {
   os.rm(filename)
 }
 
+// test_write_and_read_bytes checks for regressions made in the functions
+// read_bytes, read_bytes_at and write_bytes.
+fn test_write_and_read_bytes() {
+        file_name :=  './byte_reader_writer.tst'
+        payload   :=  [`I`, `D`, `D`, `Q`, `D`]
+
+        file_write := os.create(os.realpath(file_name)) or {
+                eprintln('failed to create file $file_name')
+                return
+        }
+
+        // We use the standard write_bytes function to write the payload and
+        // compare the length of the array with the file size (have to match).
+        file_write.write_bytes(payload.data, 5)
+
+        file_write.close()
+
+        assert payload.len == os.file_size(file_name)
+
+        file_read := os.open(os.realpath(file_name)) or {
+          eprintln('failed to open file $file_name')
+          return
+        }
+
+        // We only need to test read_bytes because this function calls
+        // read_bytes_at with second parameter zeroed (size, 0).
+        red_bytes := file_read.read_bytes(5)
+
+        file_read.close()
+
+        assert red_bytes.str() == payload.str()
+
+        // We finally delete the test file.
+        os.rm(file_name)
+}
+
 fn test_create_and_delete_folder() {
   folder := './test1'
   os.mkdir(folder)