Add continuations to tortoise.

- Tortoise now has thunks, trampolines and continuations.

- Added step and play buttoms to sample page.

thunk.js

@@ -0,0 +1,70 @@
+var gcd = function (a, b) {
+  if (b === 0) {
+    return a;
+  } else {
+    return gcd(b, a % b);
+  }
+}
+
+var gcdCPS = function (a, b, cont) {
+  console.log('gcdCPS', a, b);
+  if (b === 0) {
+    console.log('final', a, b);
+    return cont(a);
+  } else {
+    var new_cont = function (v) {
+       console.log('cont', a, b, v);
+       return cont(v);
+    }
+    return gcdCPS(b, a % b, new_cont);
+  }
+}
+
+/*
+console.log(gcdCPS(259, 111, function (v) {
+  console.log('answer', v);
+  return v;
+}));
+*/
+
+var thunk = function (f, lst) {
+  return {
+    tag: 'thunk',
+    func: f,
+    args: lst
+  };
+};
+
+var thunkValue = function (x) {
+  console.log('value', x);
+  return {
+    tag: 'value',
+    val: x
+  };
+};
+
+var gcdThunk = function (a, b, cont) {
+  console.log('gcdThunk', a, b);
+  if (b === 0) {
+    console.log('final', a, b);
+    return thunk(cont, [a]);
+  } else {
+    var new_cont = function (v) {
+       console.log('cont', a, b, v);
+       return thunk(cont, [v]);
+    }
+    return thunk(gcdThunk, [b, a % b, new_cont]);
+  }
+}
+
+var trampoline = function (thk) {
+  while (true) {
+    if (thk.tag === 'value') {
+      return thk.val;
+    }
+    if (thk.tag === 'thunk') {
+      thk = thk.func.apply(null, thk.args);
+    }
+  }
+};
+console.log(trampoline(gcdThunk(259, 111, thunkValue)));