This changes they was settings (what the user provides) are handled.
Before this, settings were passed as an object and that was it.
Now, settings are treated as an array of settings objects and this adds
full support for managing multiple sets of settings in a single pattern
instance.
This is the mechanism that's used for FreeSewing's sampling which used
to be a rather hackish implementation, but now merely sets up the
relevant list of settings, and then calls `pattern.draft()` as usual.
Things to be mindful of is that parts, the store and settings themselves
are tied to each set of settings. So where they used to be an object,
they are now an array of object.
