Add a minimalistic duck typing based stream representation.

source/vibe/core/stream.d

@@ -0,0 +1,16 @@
+module vibe.core.stream;
+
+enum isInputStream(T) = __traits(compiles, {
+	T s;
+	ubyte[] buf;
+	if (!s.empty)
+		s.read(buf);
+	if (s.leastSize > 0)
+		s.read(buf);
+});
+
+enum isOutputStream(T) = __traits(compiles, {
+	T s;
+	const(ubyte)[] buf;
+	s.write(buf);
+});