Implement collections/associateBy

[LionC]

This adds an associateBy function to the collections module.

associateBy transforms an array into a Record, often used to conveniently access it by key instead of iterating via e.g. .find() afterwards:

const users = [
    { id: 'a2e', userName: 'Anna' },
    { id: '5f8', userName: 'Arnold' },
    { id: 'd2c', userName: 'Kim' },
]

const usersById = associateBy(users, it => it.id)
const someId = 'a23'
const user = usersById[someId]

associateBy was part of the original proposal (see #1065 and its links). In the discussions around it and following the merge of the initial set of functions, it seemed that there are no strong opinions against it, so I posted this PR.

[kt3k]

@LionC Nice work! LGTM