Denormalize Views

[Luegg]

Create views for each possible variance instead of only covariance. E.g. Int :> Any should result in the views +Int %> +Any and -Any %> -Int.

Also include views like +Nothing %> +_, -_ %> -Nothing and /_ %> /<unknown>.

This refactoring offers the following benefits:

• Simplifies the view index. No need to check the variance of the lookup key.
• More flexibility, allows unidirectional views.
• Delegates most aspects of views to the feature extraction (see also #6). The index must not know about from what definition a view is derived from.