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).

test/unit/stdlib/io/multireader.c3

@@ -0,0 +1,21 @@
+module std::io @test;
+
+fn void! test_multireader()
+{
+	MultiReader mr;
+	mr.temp_init(
+		&&wrap_bytes("foo"),
+		&&wrap_bytes(" "),
+		&&wrap_bytes("bar"),
+		&&wrap_bytes("!"),
+	);
+	defer mr.free();
+
+	ByteWriter w;
+	io::copy_to(&mr, w.temp_init())!;
+	
+	String want = "foo bar!";
+	assert(w.str_view() == want,
+		"invalid data read; got: %s, want: %s", w.str_view(), want);
+}
+

test/unit/stdlib/io/multiwriter.c3

@@ -0,0 +1,17 @@
+module std::io @test;
+
+fn void! test_multiwriter()
+{
+	ByteWriter w1, w2;
+	MultiWriter mw;
+	mw.temp_init(w1.temp_init(), w2.temp_init());
+	defer mw.free();
+
+	String want = "foobar";
+	io::copy_to(ByteReader{}.init(want), &mw)!;
+	
+	assert(w1.str_view() == want,
+		"invalid write; got: %s, want: %s", w1.str_view(), want);
+	assert(w2.str_view() == want,
+		"invalid write; got: %s, want: %s", w2.str_view(), want);
+}

test/unit/stdlib/io/teereader.c3

@@ -0,0 +1,17 @@
+module std::io @test;
+
+fn void! test_teereader()
+{
+	String want = "foobar";
+
+	ByteWriter w;
+	TeeReader r = tee_reader(ByteReader{}.init(want), w.temp_init());
+
+	char[16] buf;
+	usz n = r.read(buf[..])!;
+
+	String got = w.str_view();
+	assert(n == want.len, "teereader: invalid length");
+	assert(got == want, "teereader: got: %s, want: %s", got, want);
+	assert(got == (String)buf[:n], "teereader: got: %s, want: %s", got, (String)buf[:n]);
+}