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feat(core): Implement reduce for zippers (#88)

Adds the following methods in `zip` (they are curried):
```javascript
declare function reduce<T>(
  fn: (acc: T, item: any) => T,
  initialAcc: T,
  zipper: Zipper
): T
```
```javascript
declare function reducePre<T>(
  fn: (acc: T, item: any) => T,
  initialAcc: T,
  zipper: Zipper
): T
```
These functions preform a reduction of the zipper tree in post (`reduce`) or pre (`reducePre`) order.

Also, the following method is added in `data`:
```javascript
declare function when<T>(
  pred: (item: any) => Boolean),
  fn: (acc: T, item: any) => T
): (acc: T, item:any) => T
```
It helps writing zip reducing using a predicate.

Closes #82

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