Add a `.reduce(accum, initialData)` method to accompany `.map()` and `.forEach()` about enzyme

[ljharb]

We should probably implement all the Array iteration methods, including some/every, reduceRight, filter, find, findIndex, and includes.

[lelandrichardson]

I like including some and every. reduceRight obviously if we're talking about reduce.

find is already in use and has a particular meaning (similar to jquery's find).

includes i'm slightly wary of because it's so similar sounding to contains, which would be used quite differently.

The problem we have is there are generally two "sets" of data that we are talking about here. we are talking about the explicit set of nodes being wrapped this.nodes, and then we are sometimes talking about those nodes AND all of their subtrees.

.filter() operates on the nodes
.find() operates on the nodes + their subtrees
.contains() operates on the nodes + their subtrees
.forEach() operates on the nodes
.map() operates on the nodes

[lelandrichardson]

PR Opened.