add Buffer ruby class

runtime/lib/buffer.rb

@@ -0,0 +1,17 @@
+class Buffer
+  def initialize
+    @contents = [
+      "Welcome to jes.",
+      "Press <Esc> to exit (for now!)",
+      "0123456789",
+    ]
+  end
+
+  def size
+    @contents.size
+  end
+
+  def [](index)
+    @contents[index]
+  end
+end