Add unfold

Similar to Ramda's unfold

snippets/unfold.md

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+### unfold
+
+Builds an array, using an iterator function and an initial seed value.
+
+Use a `while` loop and `Array.push()` to call the function repeatedly until it returns `false`.
+The iterator function accepts one argument (`seed`) and must always return an array with two elements ([`value`, `nextSeed`]) or `false` to terminate.
+
+```js
+const unfold = (fn, seed) => {
+  let result = [],
+    val = [null, seed];
+  while ((val = fn(val[1]))) result.push(val[0]);
+  return result;
+};
+```
+
+```js
+var f = n => (n > 50 ? false : [-n, n + 10]);
+unfold(f, 10); // [-10, -20, -30, -40, -50]
+```