In the app I'm working on I've wished that index_by had a buddy that would
assign the hash value instead of the key multiple times.

Enter index_with. Useful when building a hash from a static list of
symbols. Before you'd do:

```ruby
POST_ATTRIBUTES.map { |attr_name| [ attr_name, public_send(attr_name) ] }.to_h
```

But now that's a little clearer and faster with:

````ruby
POST_ATTRIBUTES.index_with { |attr_name| public_send(attr_name) }
```

It's also useful when you have an enumerable that should be converted to a hash,
but you don't want to muddle the code up with the overhead that it takes to create
that hash. So before, that's:

```ruby
WEEKDAYS.each_with_object(Hash.new) do |day, intervals|
  intervals[day] = [ Interval.all_day ]
end
```

And now it's just:

```ruby
WEEKDAYS.index_with([ Interval.all_day ])
```

It's also nice to quickly get a hash with either nil, [], or {} as the value.