Added various extensions.

Added coding style helpers.
Unit tests.

SystemExtensions.NetStandard/Extensions/StreamExtensions.cs

@@ -0,0 +1,40 @@
+using System.IO;
+
+namespace System.Extensions
+{
+    public static class StreamExtensions
+    {
+        public static void DoWhileReading(this Stream stream, Action<int, byte[]> action, int bufferLength = 256)
+        {
+            if (stream is null) throw new ArgumentNullException(nameof(stream));
+            if (action is null) throw new ArgumentNullException(nameof(action));
+
+            var buffer = new byte[bufferLength];
+            var read = stream.Read(buffer, 0, bufferLength);
+            while(read > 0)
+            {
+                action(bufferLength, buffer);
+                read = stream.Read(buffer, 0, bufferLength);
+            }
+        }
+
+        public static byte[] ReadBytes(this Stream stream, int count)
+        {
+            if (stream is null) throw new ArgumentNullException(nameof(stream));
+
+            var buffer = new byte[count];
+            stream.Read(buffer, 0, count);
+            return buffer;
+        }
+
+        public static Stream Rewind(this Stream stream)
+        {
+            if (stream is null) throw new ArgumentNullException(nameof(stream));
+
+            if (!stream.CanSeek) throw new InvalidOperationException("Stream doesn't support rewinding");
+
+            stream.Seek(0, SeekOrigin.Begin);
+            return stream;
+        }
+    }
+}