Nullable and usings

[drewnoakes]

• Fix .editorconfig to use tabs consistently in VS
• Specify imports and aliases as MSBuild items to reduce boilerplate in code
• Enable nullable reference types in the core library so that consumers can identify when values are null more easily
• Bump compiler version

Each step is a separate commit, so we can back out individual changes if needed.

[jingwood]

@drewnoakes Thank you very much!