Move IO into a separate module.

builtin/README.md

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+The Wren scripts in this directory get converted to C string literals and then
+inserted into their respective .c files so that the interpreter can load them
+directly without having to do any file IO.
+
+The script that does this copying is `script/generate_builtins.py`.
+
+You can invoke using `make builtin`.

builtin/core.wren

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+class List {
+  toString {
+    var result = "["
+    for (i in 0...this.count) {
+      if (i > 0) result = result + ", "
+      result = result + this[i].toString
+    }
+    result = result + "]"
+    return result
+  }
+}

builtin/io.wren

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+class IO {
+  static print(obj) {
+    IO.writeString_(obj.toString)
+    IO.writeString_("\n")
+    return obj
+  }
+
+  static write(obj) {
+    IO.writeString_(obj.toString)
+    return obj
+  }
+}