os: add basic OS functionality

testdata/stdlib.go

@@ -0,0 +1,12 @@
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	fmt.Println("stdin: ", os.Stdin.Fd())
+	fmt.Println("stdout:", os.Stdout.Fd())
+	fmt.Println("stderr:", os.Stderr.Fd())
+}

testdata/stdlib.txt

@@ -0,0 +1,3 @@
+stdin:  0
+stdout: 1
+stderr: 2