Remove the concept of custom positioning

[NickEntin]

Paralayout currently supports custom positioning via conformance to the AlignmentPositionAdjusting protocol. This can lead to some less-than-obvious behavior for consumers, which goes against the core principles behind Paralayout. As far as I know, this feature has very little usage outside of Label, which we have already proposed removing (#19), while adding complexity to our alignment logic. I propose that we remove the concept of custom positioning from the framework entirely.

If my assumption is wrong and you are using custom positioning, please speak up. I am happy to discuss leaving it in there, either in its current form or with some changes, if it's still providing significant value.

[NickEntin]

This should be fairly straightforward to address. We'll need to remove the AlignmentPositionAdjusting protocol and anywhere it's used.

[NickEntin]

There will be some other API that can be cleaned up alongside removing this as well. For example:

• point(inBoundsAt:) can be removed, since it will be the same as point(at:)
• contentSize(thatFits:) can be removed, since it will be the same as frameSize(thatFits:)
• frameContentSize can be removed, since it will be the same as frame.size