- add missing `[NotNull]` attributes
- remove now-unneeded usings for ...ModelBinding.Internal namespace
Also get ModelBinding test project working under CoreCLR
- create separate k10 and net45 folders under test in VS
- add `[InternalsVisibleTo]` to make `TypeExtensions` visible to test assembly
- correct tests' `CultureInfo` use to compile under CoreCLR
- ifdef out CoreCLR tests depending on Moq, ReadOnly, ...
- remove a couple of tests expecting TypeConverter to work