Separate SkeletonPatterns into its own key

[zbraniecki]

Part of #257 which will make #519 easier.

It also does this:

[zbraniecki]

There are no functional changes here. I moved provider::helpers to be provider::date_time since that's what it is really and introduced a temporary structure that lazily loads patterns/skeletons as needed. I think this architecture will pay off as we continue splitting data.

The rest is basically splitting patterns in multiple places into patterns and skeletons.

[gregtatum]

My biggest pieces of feedback are on:

• The naming of the "pattern" data payload being confusing, as it is not using the word "lengths".
• Under documentation of the PatternSelector machinery. It was hard to read and understand the code, and it would be helpful to have that in place. Without documentation, I feel that these make understanding and maintaining the code worse for future contributors.

If you don't want to make the changes here, please file follow-ups.

[zbraniecki]

I accidentally merged it without follow up cleanups:

• ebff263
• 5da3780

Apologies everyone!