Add Stdin.readByte().

test/io/stdin/read_byte.wren

@@ -0,0 +1,21 @@
+import "io" for Stdin
+
+for (i in 1...13) {
+  System.print(Stdin.readByte())
+}
+
+// stdin: first
+// expect: 102
+// expect: 105
+// expect: 114
+// expect: 115
+// expect: 116
+// expect: 10
+
+// stdin: second
+// expect: 115
+// expect: 101
+// expect: 99
+// expect: 111
+// expect: 110
+// expect: 100