Remove cruft, add to TestLib

I Removed the many many convenience initializers on Entity and Document. They appeared to be causing the compiler strife and there was a confusing number of them available at any one time. The recommendation is to provide convenience initializers on a per-need basis instead if they offer a lot of utility to you; you can even copy the existing convenience accessors right out of the library (they are just commented out).

I also added the ability to initialize nullable attributes with literals when their wrapped type could also be initialized from that literal.