added ewrite_buf and eputs

tests/eputs.expected

@@ -0,0 +1 @@
+hello stderr

tests/eputs.sl

@@ -0,0 +1,7 @@
+import ../stdlib/stdlib.sl
+import ../stdlib/io.sl
+
+word main
+    "hello stderr" eputs
+    0
+end

tests/eputs.test

@@ -0,0 +1 @@
+python main.py tests/eputs.sl -o /tmp/eputs > /dev/null && /tmp/eputs 2>&1