Minor internal changes

Remove sizeToFit() from SizeCalculable protocol.
This change ensure that PinLayout pin.sizeToFit() method behave correctly. As per the iOS documentation, we should not directly override sizeToFit() but rather always only implement sizeThatFits(_:) for auto-sizing needs. This update aim to remove the sizeToFit() requirement in the SizeCalculable protocol.

• Added by Antoine Lamy in Pull Request #164