`find*` methods should return optional element

[kylef]

This is a breaking change and I would wait a bit before we do this to settle the current unreleased changes.

We should rename the current find* methods to filter* because in JavaScript find methods are expected to return the single element and our implementation if the method is inconsistent. We should implement find* to return first matching item.

Take the following example:

// JS Array `find`
["a", "b", "c"].find(c => c === "b"); // "b"

// Array Element `find`
ArrayElement(["a", "b", "c"]).find(c => c.toValue() === "b"); // ["b"]
// We return array slice of B not just B.