Avoid hash collisions by including more keys

[geirsagberg]

We had a problem just like #22, seemingly due to hash collisions.

This PR expands the instance cache to use a Tuple with identifier hash, type hash and optional parent reference, to avoid any hash collisions.
Also:

• Wrap hash arithmetic in unchecked to avoid arithmetic overflow
• Make most types internal
• Add optional parameter keepCache to Map and DynamicMap methods. This fixes issues we had when calling similar queries sequentially in same HttpContext, e.g. when grouping by a field it would aggregate the collection entries, so first query returned 10 children, second returned 20, third 30 etc.