To filter types, add specification classes for types

[craigfowler]

This issue was initially blocked by https://github.com/csf-dev/CSF.Data/issues/15 - which has now been resolved.

I'd like to create some reflection-based specification implementations. These should be as follows:

• A specification for concrete classes
• A specification for open generics
• A specification for types which derive from another type
• A specification for interfaces

[craigfowler]

There are notes for how to do this pasted into the comments for https://github.com/csf-dev/CSF.Reflection/issues/5