Add io stream primitives (#1626)

* io: implement MultiReader struct

Implement a MultiReader (InStream) which sequentially read from the
provided readers (InStreams). Return IoError.EOF when all of the readers
are read.

* io: implement MultiWriter struct

Implement a MultiWriter (OutStream). The MultiWriter duplicates its
writes to all the provided writers (OutStream).

* io: implement TeeReader struct

Implement a TeeReader (InStream) which reads from a wrapped reader
(InStream) and writes data to the provided writer (OutStream).

lib/std/io/stream/teereader.c3

@@ -0,0 +1,42 @@
+module std::io;
+
+struct TeeReader (InStream)
+{
+	InStream r;
+	OutStream w;
+}
+
+<* Returns a reader that implements InStream and that will write any data read
+ from the wrapped reader r to the writer w. There is no internal buffering.
+ 
+ @param [&inout] r "Stream r to read from."
+ @param [&inout] w "Stream w to write to what it reads from r."
+*>
+macro TeeReader tee_reader(InStream r, OutStream w) => { r, w };
+
+<*
+ @param [&inout] self
+ @param [&inout] r "Stream r to read from."
+ @param [&inout] w "Stream w to write to what it reads from r."
+ *>
+fn TeeReader* TeeReader.init(&self, InStream r, OutStream w)
+{
+	*self = tee_reader(r, w);
+	return self;
+}
+
+fn usz! TeeReader.read(&self, char[] bytes) @dynamic
+{
+	usz nr, nw;
+	nr = self.r.read(bytes)!;
+	nw = self.w.write(bytes[:nr])!;
+	if (nr != nw) return IoError.GENERAL_ERROR?;
+	return nr;
+}
+
+fn char! TeeReader.read_byte(&self) @dynamic
+{
+	char[1] data;
+	self.read(data[..])!;
+	return data[0];
+}