Refactor InterfaceType to support partial at the property level

[untio11]

From review comment:

I think we should consider refactoring InterfaceType to support that as well. All the ingredients are already in place. This could have the following benefits: (I haven't tried it though)

• No more need for a seperate PickType, we can now delegate entirely to InterfaceType (sometimes combined with a UnionType when the input type is a union) from within the pick function
• withOptional then returns an InterfaceType instead of an IntersectionType makes sense I guess?¹
• we can support config options like strictMissingKeys etc
  If we decide to do that, we should do so in another branch. Let's discuss that offline.

Footnotes

1. Currently, objects with both mandatory and optional properties are implemented as an intersection of one object with the mandatory properties and one object with the optional properties. ↩